Also when I try to enter it manually I get the message "The user is not known to the underlying authentication module".
I don't know exactly the cause but I found a workaround.
Normally you just have to connect to the network and then login manually (Like you do the first time when you use AD integration).
Easy when you are in reach of your network...
But I also found another way, that allows to restore everything when you are offline.
Just create a backup (as root) of your /var/lib/samba directory
And when you have this problem, just unpack this backup like this (also as root)
tar -cvzf /var/lib/samba.tar.gz /var/lib/samba/
So it seems like a corrupted cache but nothing in the logfiles given me any clue.
cd /var/lib tar --overwrite -xvzf samba.tar.gz
Maybe I should create a bug report but until then, this solution also works...