For those useres affected, we could fix the problem by setting the roundcube standard date format from the German to the Englisch version. Then the save button works again. Nevertheless, those users, who did not have this problem, continue to use the German date format setting without a problem. All this on the same server and same domain.

The date and time format under Settings in Roundcube can be identical between user that have this problem and user that don't. At least at this level that does not seem to make a difference.

I can confirm this.
This was working fine. Some update changed the situation.
Most users cannot create new events in their calendar. one however still can.
This is in roundcube. Event creation via caldav is not affected.

If there is a patch for Winterfell could you please port this also to Kolab16?
I have a current Kolab16 installation in a Centos7 lxc container on an Ubuntu 16.04 host.
kolab lm is not working and there are no folders created for new users.

I have the same syptoms on a fully updated kolab16 on centos7.
Rebooting the sever starts things going for a while until ldap is blocked again.
If it happens Roundcube login does not work and sending mails via kontact is blocked.