Spending a weeks to troubleshoot the "Sys.WebForms.PageRequestManagerParserErrorException" error in my company’s project.
Finally I found the workaround for it. Very easy, just disable the enableEventValidation on the ASPX page containing the UpdatePanel.
If this workaround doesn’t work for you, you should probably look @ Eilon Lipton’s Blog. Here is some of his findings:
Why do I keeping getting a PageRequestManagerParserErrorException?