diff --git a/backend/src/apis/humhub/compareHumhubUserDbUser.test.ts b/backend/src/apis/humhub/compareHumhubUserDbUser.test.ts
index cc636d17a..a3d6c7044 100644
--- a/backend/src/apis/humhub/compareHumhubUserDbUser.test.ts
+++ b/backend/src/apis/humhub/compareHumhubUserDbUser.test.ts
@@ -1,4 +1,7 @@
 /* eslint-disable prettier/prettier */
+// eslint-disable-next-line import/no-unassigned-import
+import 'reflect-metadata'
+import { PublishNameType } from '@/graphql/enum/PublishNameType'
 import { communityDbUser } from '@/util/communityUser'
 
 import { isHumhubUserIdenticalToDbUser } from './compareHumhubUserDbUser'
@@ -13,6 +16,7 @@ describe('isHumhubUserIdenticalToDbUser', () => {
         defaultUser.alias = 'alias'
         defaultUser.emailContact.email = 'email@gmail.com'
         defaultUser.language = 'en'
+        defaultUser.gradidoID = 'gradidoID'
     })
 
     it('Should return true because humhubUser was created from entity user', () => {
@@ -21,6 +25,20 @@ describe('isHumhubUserIdenticalToDbUser', () => {
         expect(result).toBe(true)
     })
 
+    it('Should return false, because last name differ because of publish name type', () => {
+        const humhubUser = new GetUser(defaultUser, 1)
+        defaultUser.humhubPublishName = PublishNameType.PUBLISH_NAME_FIRST
+        const result = isHumhubUserIdenticalToDbUser(humhubUser, defaultUser)
+        expect(result).toBe(false)
+    })
+
+    it('Should return true, even if alias is empty', () => {
+        defaultUser.alias = ''
+        const humhubUser = new GetUser(defaultUser, 1)
+        const result = isHumhubUserIdenticalToDbUser(humhubUser, defaultUser)
+        expect(result).toBe(true)
+    })
+
     it('Should return false because first name differ', () => {
         const humhubUser = new GetUser(defaultUser, 1)
         humhubUser.profile.firstname = 'changed first name'
diff --git a/backend/src/apis/humhub/syncUser.ts b/backend/src/apis/humhub/syncUser.ts
index 8b7963b07..e5b793ee3 100644
--- a/backend/src/apis/humhub/syncUser.ts
+++ b/backend/src/apis/humhub/syncUser.ts
@@ -22,6 +22,10 @@ function isValid(postUser: PostUser, userId: number): boolean {
     logger.error('firstname too long for humhub, for user with id:', userId)
     return false
   }
+  if (postUser.profile.lastname.length > 20) {
+    logger.error('lastname too long for humhub, for user with id:', userId)
+    return false
+  }
   return true
 }
 
diff --git a/backend/src/data/PublishName.logic.ts b/backend/src/data/PublishName.logic.ts
index a78627d49..b70a29bb3 100644
--- a/backend/src/data/PublishName.logic.ts
+++ b/backend/src/data/PublishName.logic.ts
@@ -35,12 +35,16 @@ export class PublishNameLogic {
    * @returns user.firstName for PUBLISH_NAME_FIRST, PUBLISH_NAME_FIRST_INITIAL or PUBLISH_NAME_FULL
    */
   public getFirstName(publishNameType: PublishNameType): string {
+    let firstName = ''
+    if (this.user && typeof this.user.firstName === 'string') {
+      firstName = this.user.firstName
+    }
     return [
       PublishNameType.PUBLISH_NAME_FIRST,
       PublishNameType.PUBLISH_NAME_FIRST_INITIAL,
       PublishNameType.PUBLISH_NAME_FULL,
     ].includes(publishNameType)
-      ? this.user.firstName
+      ? firstName.slice(0, 20)
       : ''
   }
 
@@ -51,10 +55,14 @@ export class PublishNameLogic {
    *   first initial from user.lastName for PUBLISH_NAME_FIRST_INITIAL
    */
   public getLastName(publishNameType: PublishNameType): string {
+    let lastName = ''
+    if (this.user && typeof this.user.lastName === 'string') {
+      lastName = this.user.lastName
+    }
     return publishNameType === PublishNameType.PUBLISH_NAME_FULL
-      ? this.user.lastName
-      : publishNameType === PublishNameType.PUBLISH_NAME_FIRST_INITIAL
-      ? this.user.lastName.charAt(0)
+      ? lastName.slice(0, 20)
+      : publishNameType === PublishNameType.PUBLISH_NAME_FIRST_INITIAL && lastName.length > 0
+      ? lastName.charAt(0)
       : ''
   }