diff --git a/alfa-client/libs/admin-settings/src/lib/postfach/postfach-container/postfach-form/postfach.formservice.ts b/alfa-client/libs/admin-settings/src/lib/postfach/postfach-container/postfach-form/postfach.formservice.ts
index 26fe9f14a62bfb70ba23927fded324682995ac62..d8071b1434a873de385f57eceb81b720a4e06eec 100644
--- a/alfa-client/libs/admin-settings/src/lib/postfach/postfach-container/postfach-form/postfach.formservice.ts
+++ b/alfa-client/libs/admin-settings/src/lib/postfach/postfach-container/postfach-form/postfach.formservice.ts
@@ -1,4 +1,9 @@
-import { AbstractFormService, HttpError, StateResource } from '@alfa-client/tech-shared';
+import {
+  AbstractFormService,
+  EMPTY_STRING,
+  HttpError,
+  StateResource,
+} from '@alfa-client/tech-shared';
 import { Injectable } from '@angular/core';
 import { FormControl, UntypedFormBuilder, UntypedFormGroup } from '@angular/forms';
 import { Observable } from 'rxjs';
@@ -27,13 +32,13 @@ export class PostfachFormService extends AbstractFormService {
   protected initForm(): UntypedFormGroup {
     return this.formBuilder.group({
       [PostfachFormService.ASBSENDER_GROUP]: this.formBuilder.group({
-        [PostfachFormService.NAME_FIELD]: new FormControl(''),
-        [PostfachFormService.ANSCHRIFT_FIELD]: new FormControl(''),
-        [PostfachFormService.DIENST_FIELD]: new FormControl(''),
-        [PostfachFormService.MANDANT_FIELD]: new FormControl(''),
-        [PostfachFormService.GEMEINDESCHLUESSEL_FIELD]: new FormControl(''),
+        [PostfachFormService.NAME_FIELD]: new FormControl(EMPTY_STRING),
+        [PostfachFormService.ANSCHRIFT_FIELD]: new FormControl(EMPTY_STRING),
+        [PostfachFormService.DIENST_FIELD]: new FormControl(EMPTY_STRING),
+        [PostfachFormService.MANDANT_FIELD]: new FormControl(EMPTY_STRING),
+        [PostfachFormService.GEMEINDESCHLUESSEL_FIELD]: new FormControl(EMPTY_STRING),
       }),
-      [PostfachFormService.SIGNATUR_FIELD]: new FormControl(''),
+      [PostfachFormService.SIGNATUR_FIELD]: new FormControl(EMPTY_STRING),
     });
   }
 
diff --git a/alfa-client/libs/tech-shared/src/lib/error/error.util.ts b/alfa-client/libs/tech-shared/src/lib/error/error.util.ts
index c6029db8c9e7534811804cf9ed2e3ce929997b73..81c2a8d62e817d5e4907c194bf9dd62bfa2fd3a1 100644
--- a/alfa-client/libs/tech-shared/src/lib/error/error.util.ts
+++ b/alfa-client/libs/tech-shared/src/lib/error/error.util.ts
@@ -22,8 +22,9 @@
  * unter der Lizenz sind dem Lizenztext zu entnehmen.
  */
 import { HttpErrorResponse } from '@angular/common/http';
-import { ApiError, MessageCode } from '@alfa-client/tech-shared';
 import { isNil } from 'lodash-es';
+import { MessageCode } from '../message-code';
+import { ApiError, HttpError } from '../tech.model';
 
 export function isApiError(value: any): boolean {
   return !isNil(value?.issues) && !isNil(value.issues[0]);
diff --git a/alfa-client/libs/tech-shared/src/lib/validation/tech.validation.util.spec.ts b/alfa-client/libs/tech-shared/src/lib/validation/tech.validation.util.spec.ts
index 658c577bd6ab2d7213a6288657c47e0a0aeef500..9e0902270b2423a9510872b1a5f15828549eb8d6 100644
--- a/alfa-client/libs/tech-shared/src/lib/validation/tech.validation.util.spec.ts
+++ b/alfa-client/libs/tech-shared/src/lib/validation/tech.validation.util.spec.ts
@@ -21,7 +21,13 @@
  * Die sprachspezifischen Genehmigungen und Beschränkungen
  * unter der Lizenz sind dem Lizenztext zu entnehmen.
  */
-import { AbstractControl, FormControl, FormGroup, UntypedFormControl, UntypedFormGroup } from '@angular/forms';
+import {
+  AbstractControl,
+  FormControl,
+  FormGroup,
+  UntypedFormControl,
+  UntypedFormGroup,
+} from '@angular/forms';
 import { createInvalidParam, createIssue } from '../../../test/error';
 import { InvalidParam, Issue } from '../tech.model';
 import {
@@ -145,9 +151,9 @@ describe('ValidationUtils', () => {
   });
 
   describe('invalid param', () => {
-    const formPrefixes = ['', 'some-prefix'];
-    const fieldNames = ['baseField1', 'baseField2', 'subGroup.subGroupField1'];
-    const prefixNameCombinations = formPrefixes.flatMap((prefix) =>
+    const formPrefixes: string[] = ['', 'some-prefix'];
+    const fieldNames: string[] = ['baseField1', 'baseField2', 'subGroup.subGroupField1'];
+    const prefixNameCombinations: string[][] = formPrefixes.flatMap((prefix) =>
       fieldNames.map((name) => [prefix, name]),
     );
     const unknownName = 'unknown-field';
@@ -183,11 +189,11 @@ describe('ValidationUtils', () => {
 
         describe('set invalid param validation error', () => {
           it('should assign invalidParam to form control error without prefix', () => {
-            const message = getMessageForInvalidParam(invalidParam, prefix);
+            const message: string = getMessageForInvalidParam(invalidParam, prefix);
 
             setInvalidParamValidationError(form, invalidParam, prefix);
 
-            const errorMessage = form.getError(invalidParam.reason, fieldName);
+            const errorMessage: string = form.getError(invalidParam.reason, fieldName);
             expect(errorMessage).toBe(message);
           });
 
diff --git a/alfa-client/libs/tech-shared/src/lib/validation/tech.validation.util.ts b/alfa-client/libs/tech-shared/src/lib/validation/tech.validation.util.ts
index a42904b4cc50a322b0345f71df54f4c7824da733..4a34005de2f231027678dc6ce606a8cc2e3e1bed 100644
--- a/alfa-client/libs/tech-shared/src/lib/validation/tech.validation.util.ts
+++ b/alfa-client/libs/tech-shared/src/lib/validation/tech.validation.util.ts
@@ -83,7 +83,7 @@ export function getMessageCode(apiError: ApiError): string {
 export function setInvalidParamValidationError(
   form: UntypedFormGroup,
   invalidParam: InvalidParam,
-  pathPrefix: string = '',
+  pathPrefix?: string,
 ): void {
   const control: AbstractControl = getControlForInvalidParam(form, invalidParam, pathPrefix);
   if (isNotNil(control)) {
@@ -94,14 +94,10 @@ export function setInvalidParamValidationError(
   }
 }
 
-function getFieldPathWithoutPrefix(name: string, pathPrefix: string): string {
-  return pathPrefix.length > 0 ? name.substring(pathPrefix.length + 1) : name;
-}
-
 export function getControlForInvalidParam(
   form: UntypedFormGroup,
   invalidParam: InvalidParam,
-  pathPrefix: string = '',
+  pathPrefix?: string,
 ): AbstractControl {
   return form.get(getFieldPathWithoutPrefix(invalidParam.name, pathPrefix));
 }
@@ -113,3 +109,7 @@ export function getMessageForInvalidParam(item: InvalidParam, pathPrefix: string
     getFieldPathWithoutPrefix(item.name, pathPrefix),
   );
 }
+
+function getFieldPathWithoutPrefix(name: string, pathPrefix?: string): string {
+  return pathPrefix ? name.substring(pathPrefix.length + 1) : name;
+}
diff --git a/alfa-client/libs/ui/src/lib/ui/editor/formcontrol-editor.abstract.component.ts b/alfa-client/libs/ui/src/lib/ui/editor/formcontrol-editor.abstract.component.ts
index 345cf7de9e8bc26499a742b344023b7b0ec018d2..517a4a67c60e32e79a882f4c84e2c9e4565e333b 100644
--- a/alfa-client/libs/ui/src/lib/ui/editor/formcontrol-editor.abstract.component.ts
+++ b/alfa-client/libs/ui/src/lib/ui/editor/formcontrol-editor.abstract.component.ts
@@ -80,7 +80,7 @@ export abstract class FormControlEditorAbstractComponent implements ControlValue
     if (this.statusSubscr) this.statusSubscr.unsubscribe();
   }
 
-  setErrors() {
+  setErrors(): void {
     if (this.control) {
       this.fieldControl.setErrors(this.control.errors);
       if (this.control.invalid) this.fieldControl.markAsTouched();
diff --git a/alfa-client/libs/user-profile/src/lib/user-icon/user-icon.component.ts b/alfa-client/libs/user-profile/src/lib/user-icon/user-icon.component.ts
index 82976c7be94d3b9cec56f1b68079815d2cbf9352..78547141a5665a15799b97afc211cee0134f8d49 100644
--- a/alfa-client/libs/user-profile/src/lib/user-icon/user-icon.component.ts
+++ b/alfa-client/libs/user-profile/src/lib/user-icon/user-icon.component.ts
@@ -22,7 +22,13 @@
  * unter der Lizenz sind dem Lizenztext zu entnehmen.
  */
 import { Component, Input, SimpleChanges } from '@angular/core';
-import { createEmptyStateResource, hasError, MessageCode, StateResource } from '@alfa-client/tech-shared';
+import {
+  ApiError,
+  createEmptyStateResource,
+  hasError,
+  MessageCode,
+  StateResource,
+} from '@alfa-client/tech-shared';
 import {
   getUserName,
   getUserNameInitials,
@@ -71,7 +77,7 @@ export class UserIconComponent {
   }
 
   get errorMessageCode(): string {
-    return this.userProfileStateResource.error?.issues[0]?.messageCode;
+    return (<ApiError>this.userProfileStateResource.error)?.issues[0]?.messageCode;
   }
 
   get initials(): string {
diff --git a/alfa-client/libs/user-profile/src/lib/user-profile-in-vorgang-container/user-profile-in-vorgang/user-profile-in-vorgang.component.ts b/alfa-client/libs/user-profile/src/lib/user-profile-in-vorgang-container/user-profile-in-vorgang/user-profile-in-vorgang.component.ts
index a255e3a75afd20a9085cae37f92e405d00fc0fd8..4a93a23e00e7211c40a49c585acd73d78bba2ed7 100644
--- a/alfa-client/libs/user-profile/src/lib/user-profile-in-vorgang-container/user-profile-in-vorgang/user-profile-in-vorgang.component.ts
+++ b/alfa-client/libs/user-profile/src/lib/user-profile-in-vorgang-container/user-profile-in-vorgang/user-profile-in-vorgang.component.ts
@@ -23,6 +23,7 @@
  */
 import { Component, Input } from '@angular/core';
 import {
+  ApiError,
   StateResource,
   createEmptyStateResource,
   hasError,
@@ -45,7 +46,7 @@ export class UserProfileInVorgangComponent {
   readonly vorgangLinkRel = VorgangWithEingangLinkRel;
 
   public isUserServiceAvailable(stateResource: StateResource<Resource>): boolean {
-    if (hasError(stateResource) && isServiceUnavailableMessageCode(stateResource.error)) {
+    if (hasError(stateResource) && isServiceUnavailableMessageCode(<ApiError>stateResource.error)) {
       return false;
     }
     return true;