We've completed our investigation of the intermittent exceptions being reported by FogBugz On Demand customers this morning.
The exception is for the URL Trigger plug-in, which is installed by default on FogBugz On Demand accounts.
The exception is/was totally benign and should no longer appear. It was cleared by a simple recycling of the application pools on the various web servers.
We are keeping an eye out for the same and similar errors in the future, but do not expect this to recur.