Update: Thanks to WordPress.com automagically linking related posts to each other, I think I may have found a potential solution to this issue here: Invalid character in a Base-64 string (ASP.NET)
The problem seems to be with the ViewState. The ViewState is encrypted, and when an attempt is made to decrypt it on postback, the error is triggered. The solution is actually quite simple: in the web.config file, set the ViewState not to be encrypted, like this:<system.web>
You should really check out the rest of that article; it looks like it’s got a great explanation for what is actually occurring.
Also, this seems to be related to another message, “Invalid Viewstate.”
My original article is below the fold…