Hello, Ingo!
Is it possible that the user is requesting the license against the URL of the floating seats group, instead of the URL of the group for managed seats? This seems like something that would explain the issue.
In this case you can send him a new URL to connect to the License Server. You can find Group URLs from the License Server Groups tab.
If that is not the case then please send us jrebel-licensing.log, jrebel-{ide}.log from your user’s machine and, if possible, application.log from license server installation directory. You can send the logs to support@zeroturnaround.com
Vita