Every ASP.NET page has at least one hidden field named "VIEWSTATE" and several pages have one or two others. If you look at the Page Source in your browser, you will see that the hidden field values are very long strings of gobbledygook. That gobbledygook is Base 64 encoded data. When the page is posted to the server, ASP.NET tries to unencode the string. If the string is corrupted in any way, ASP.NET will record the error you reported.

So the error means that one of the hidden fields is corrupted.

Since the VIEWSTATE hidden field is nearly always the longest string, it is most likely the corrupted string.

My crystal ball is not working, so I can't tell you why the string is corrupted. But my guess is that you have selected the option to move ViewState to the end of the page and people are clicking on an element that posts the page before the VIEWSTATE hidden field is fully rendered.

"Will this bring down the site..."

No.

"...or show an error page?"

Probably.

"How can I find that page?"

It can be any page. If you have checked the option to move the ViewState to the bottom of the page, then turn that off. If you did not check that, then talk to your host to see if they have any ideas about what could be causing the problem.

"Is it in the CSS?"

No.

Marcy, you had the setting selected to move the view state to the bottom of the page. I removed this as it has been known to cause issue. Clear your logs and test to see if it comes back.

Let me know.

Admin Area > Options > Performance . The item is located towards the bottom of the page.

ScanAlert purposefully tries to hack your site and break it in an attempt to find vulnerabilities. It's pretty normal that your event log will be riddled with errors after ScanAlert performs a scan. It's pretty annoying, but there's no harm in it. As long as ScanAlert isn't reporting an vulnerabilities to you in their reports, I wouldn't worry about it.