Can't delete Virtual Client on server

There are two virtual clients which I can not delete on the Server. They are both virtual clients of the same client.
Whenever I try to delete them always the message “Inactivity caused this session to become invalid. Please login again to proceed.” appears and I need to login again.
Running the Server in Debug mode, doesn’t give any output at all.
When trying to remove the backups of one of the clients, “urbackupclientctl remove …” the command hangs forever and the backup isn’t removed.

Urbackup server: 2.5.32
OS: Debian Linux Stable