There is aparently a quite severe issue with the /region addmember.. and /region removemember commands. The database engine of the worldguard plugin does not seem to be able to save changes made by this command and basically crashes the server.
I have yet to find out what the issue is. In the meantime, I have disabled that command. If you urgently need to remove someone from your lot, please leave a comment here and I will make the change in the database.