I just upgraded my old 1.9.1.8 version of YAF to 1.9.5 and started seeing these "Validation of viewstate MAC failed" errors in the event log. On looking at the IPs a little closer I found that these were the spammers that were inundating my old version with bogus members just so they could include their bogus websites in their profiles. Evidently the newer security is instead causing and error to be thrown. I'm not sure if there is a way to turn these errors off, but to me it is much more preferable to clear the event log once in a while than to go through the bogus members and take them out.

Not a problem for me but just wanted to share with those seeing the same event log issue.