When generating a screenshot using FogBugz Screenshot tool, I get the following error:

This used to open in a new browser window, whereas now it opens in the login window and generates the error displayed.
If I close these windows and attempt to make a second screenshot, then this works fine, and continues to work fine until I next reboot my computer and try again.
Why is this happening, and is there anything I can do to prevent it?
UPDATE: I've also tested this in BugShooting, and I get the same error when sending to FogBuz. Which suggests to me that it is the FogBugz web application which is the problem.
Why is this?