Connecting to AD: failed to sync!

I’ve found a few similar threads, but no comments/hints…

When connecting Keycloak to Active Directory, both tests (connection and authentication) pass but when syncing users I get this message (tried different 2008/2012/2019 AD servers)

Any hints what to look for?

Looking at the server logs again I founds this

“User returned from LDAP has null username!”

And that pointed out the solution to the problem shown here, for example
https://edwin.baculsoft.com/2019/10/fixing-error-null-username-when-integrating-redhat-single-sign-on-to-active-directory/