>I can't say what is causing the invalid viewstate (there are some things your code can do which can cause this problem like adding controls dynamically), and some things out of your control. But I'd say the error is that the posted back viewstate was corrupt (missing data, data truncated, etc.) and when it attempted to deserialize the information in the viewstate back into the statebag it failed because some of the data it was expecting was missing.

Since I have this in the Pages declaration of the Web.config file, I didn't get that message again.

enableEventValidation="false"

I had those errors at a few occasions for two days until I added that parameter. Now, for two days, I didn't get it. I will keep an eye on this however as it might just be a coincidence.