diff --git a/backend/src/apis/humhub/ExportUsers.ts b/backend/src/apis/humhub/ExportUsers.ts
index 99c4471e5..d4515b3ce 100644
--- a/backend/src/apis/humhub/ExportUsers.ts
+++ b/backend/src/apis/humhub/ExportUsers.ts
@@ -88,7 +88,7 @@ async function main() {
   const humhubUsers = await loadUsersFromHumHub(humHubClient)
 
   let dbUserCount = 0
-  const executedHumhubActionsCount = [0, 0, 0, 0]
+  const executedHumhubActionsCount = [0, 0, 0, 0, 0]
 
   do {
     try {
@@ -121,6 +121,7 @@ async function main() {
     updatedCount: executedHumhubActionsCount[ExecutedHumhubAction.UPDATE],
     skippedCount: executedHumhubActionsCount[ExecutedHumhubAction.SKIP],
     deletedCount: executedHumhubActionsCount[ExecutedHumhubAction.DELETE],
+    validationErrorCount: executedHumhubActionsCount[ExecutedHumhubAction.VALIDATION_ERROR],
   })
 }
 
diff --git a/backend/src/apis/humhub/syncUser.ts b/backend/src/apis/humhub/syncUser.ts
index 8057626af..8b7963b07 100644
--- a/backend/src/apis/humhub/syncUser.ts
+++ b/backend/src/apis/humhub/syncUser.ts
@@ -1,6 +1,7 @@
 import { User } from '@entity/User'
 
 import { LogError } from '@/server/LogError'
+import { backendLogger as logger } from '@/server/logger'
 
 import { isHumhubUserIdenticalToDbUser } from './compareHumhubUserDbUser'
 import { HumHubClient } from './HumHubClient'
@@ -12,7 +13,18 @@ export enum ExecutedHumhubAction {
   CREATE,
   SKIP,
   DELETE,
+  VALIDATION_ERROR,
 }
+
+// todo: replace with full validation (schema)
+function isValid(postUser: PostUser, userId: number): boolean {
+  if (postUser.profile.firstname.length > 20) {
+    logger.error('firstname too long for humhub, for user with id:', userId)
+    return false
+  }
+  return true
+}
+
 /**
  * Trigger action according to conditions
  * | User exist on humhub | export to humhub allowed | changes in user data | ACTION
@@ -30,6 +42,9 @@ export async function syncUser(
   humhubUsers: Map<string, GetUser>,
 ): Promise<ExecutedHumhubAction> {
   const postUser = new PostUser(user)
+  if (!isValid(postUser, user.id)) {
+    return ExecutedHumhubAction.VALIDATION_ERROR
+  }
   const humhubUser = humhubUsers.get(postUser.account.username)
   const humHubClient = HumHubClient.getInstance()
   if (!humHubClient) {