>I'm not sure how the EnableEventValidation setting would have any effect on a 'Invalid viewstate' error.
>
>Setting EnableViewState to false would certainly stop to error since there would be no ViewState to validate <s>. But whilst you may not need to maintain ViewState on this particular page I assume there are other pages where this is not so - and those pages are probably still prone to the invalid viewstate error.
>
>I'd suggest implementing the fixed key as recommended in the link Tracy supplied.

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.