This is the same error I received before that I logged here:
http://social.technet.microsoft.com/Forums/en-US/project2010custprog/thread/02bffbc0-897f-41ba-b681-a3d0cde5e5de
Looks like the culprit was another .NET Security update. The fix was the same as what I posted in the above link... Change the master page back to default go to the root, then change back to the custom master page. This isn't the best fix, so I'll look into
why it errors out after these updates in the first place.
updates to server:

Related KB articles:
KB2690533
KB2658846
KB2656410
KB2604114
KB2688338
KB2659262
KB2676562
Anyone else run into this issue and know how to permanently fix?
Thank you in advance, Peter