we did encounter a "crash" (whatever it was) and had to reboot the ALM server.
After that, now, we have some licenses locked although there are no connections to QC.
We already had this problem in the past, and multiple reboots did solve the problem.
But this time, the licenses remain locked.
thank you for your advices. "Unfortunately", my collegue has solved the problem already. There is a "site administration database" which can be only accessed via the SQL server. Here you can see - and remove - the locks.