Steps to reproduce:

1. Login as user with LDAP authentication
2. Open "Profile" from "User Settings"
3. Click "Update" (with or without changing anything)

Result:
Error Message: "Cannot update user":

• Incorrect value for field "passwd": cannot be empty. [zabbix.php:22 → require_once() → ZBase->run() → ZBase->processRequest() → CController->run() → CControllerUserProfileUpdate->doAction() → CApiWrapper->__call() → CFrontendApiWrapper->callMethod() → CApiWrapper->callMethod() → CFrontendApiWrapper->callClientMethod() → CLocalApiClient->callMethod() → CUser->update() → CUser->validateUpdate() → CUser->checkUserGroups() → CApiService::exception() in include/classes/api/services/CUser.php:755]

(see Screenshot)

Expected:
Settings updated / no error message

Additional information:

Updating the user profile (as administrator) via "Users" > "Users" is possible. The error mentioned above also happens when the affected user is an administrator.

The issue exists (at least) since earlier 6.4 releases.