Exception in thread "pool-3-thread-36"

[ghost]

I've been having this issue for a while, and it seemed at first that it wasn't causing anything too bad. But now that I'm done fixing something else and looked into it, it's more important than I thought.
There's this error in the console I can't make much of, but I'm thinking it has something to do with the groups not getting changed. (I figured out that this error gets thrown every time I use /cbsyncall)

Here is the server log: http://pastebin.com/wn2kfBdQ
Here is my config file: http://pastebin.com/rqDQzDdT

Also, when a group does get changed (The sync works when I restart the server), it seems to be added to the player so that he is part of two groups. Though I just want to set his group instead of adding him to one!

Any help would be greatly appreciated,
Thanks in advance!