Comment 1 for bug 745083

So it's happening on the call to Branch.set_last_revision. That operation itself is very simple, but it also triggers pre/post_change_branch_tip hooks on the server. So my suspicion is that there's a plugin installed on the server that uses those hooks, and the error is originating there. Given that branch is actually updated, it's almost certainly something that occurs during the post_change_branch_tip hook.

Can you find out which plugins are installed on the server? Although probably there's a traceback in the server's log that will make it clear which one is the culprit.