Thanks for your beautifully-written problem report.
We thought the error you describe was fixed in version 2.0.1; perhaps this is a similar (but different) problem. We think it occurs on a machine that has been running for several days; this is usually a server. As you have noted, it only occurs when viewing a message with HTML format available.
The only known workaround is to restart the machine. However the error may recur if the machine has been running nonstop for 30 days or more (sometimes less).
Alternatively, we can make a special build of the Viewer available to you. The build would be capable of identifying what the Viewer is attempting to do when the error occurs. This information would be very helpful to us in understanding how to fix the problem. Please let me know if you are interested.