@ArniVidar - welcome to MobileRead.
Now that looks an interesting one, it looks like the Calibre database code is not liking the value of the ISBN that is being passed through. I have two suggestions:
(1) See if you can isolate it down to a specific book that is causing the issue. If you can do that, and give me all the info you can on it (existing ISBN/goodreads id it has if any, any info you see on screen when you click the Sync button etc) I can try to replicate it.
(2) Alternatively, what I would hope would work (but not help figure out the cause) is to go to the Customise plugin dialog and change your setting for "When linking to goodreads" to "Never modify the Calibre ISBN".
Let me know how you get on.