UCS-Backup-Directory node (Mail-Server) does not synch LDAP, Re-Join-Domain failed

Hi,

since the update to UCS 5.2 I am experiencing problems with my mail clients. The mail client periodically (multiple times daily) forget all mails. 5 Minutes later the reload all the mails from the mailserver. Mailserver itself does not show any errors (I am using the standard UCS-Mail-Server (dovecot).
I recently notes that Thunderbird throws the error message: “mailbox does not exist” in these cases.

Therefore I thought maybe it is connected to LDAP.
Looking into the protocols on the Backup-node I find a lot of messages (most common are looking like this:)

Jul 20 08:53:17 m-server slapd[1559]: get_ssa: conn 25334 unknown attribute type=sudoHost (17)
Jul 20 09:08:17 m-server slapd[1559]: get_filter: conn 25375 unknown attribute type=sudoHost (17)
Jul 20 09:11:21 m-server slapd[1559]: do_syncrepl: rid=001 rc -1 retrying (19 retries left)
Jul 20 04:38:17 m-server slapd[1559]: connection_read(29): no connection!

So I tried to re-join the domain, in order to try and correct the error.
The server could not complete the Domain join, it failed at script 04 (univention-ldap-client).
I got the following error message:
FAILED: failed to copy /var/lib/univention-ldap/notify/transaction from the Primary Directory Node. Please try again.
EDIT: I managed to complete the domain join by using the user “Administrator” instead of my normal “Domain-Admin” user. I will watch the system if the eror has been cleared up by the Re-Join.

EDIT2: The log throws a lot of lines (one for each user) like the following:
Jul 20 22:45:27 m-server slapd[5125]: conn=1000 op=74: memberof_value_modify DN="cn=domain users,cn=groups,dc=domain,dc=de" add memberOf="cn=Users,cn=Builtin,dc=domain,dc=de" failed err=32

In Addition I (now) found the following lines:
Jul 20 22:52:03 m-server slapd[896]: No trusted audiences configured
Jul 20 22:52:03 m-server slapd[896]: oauthbearer_client_plug_init() failed in sasl_server_add_plugin(): error when parsing configuration file
Jul 20 22:52:03 m-server slapd[896]: _sasl_plugin_load failed on sasl_server_plug_init for plugin: oauthbearer
Jul 20 22:52:06 m-server slapd[1233]: WARNING: No dynamic config support for overlay shadowbind.

Could somebody please point me to a solution?
(I’ll happily post logfiles, I you tell me which ones are relevant)

Versions: both UCS-machines run UCS 5.2-2 errata150