Its just creepy crawlers all over the web poking at your site. It is very easy to cause errors to be logged, but it generally has no impact on real users. You can cause this error in your log yourself simply by requesting the url http://yoursiteroot/ScriptResource.axd
so any crawler can also cause this error. It is a nuisance but no real harm is caused by it. I was getting these in my log every 3 minutes from a range of ip addresses in China, I collected all the ip addresses and banned them as it is clearly not normal traffic but some automated script.
I would not go crazy banning ip addresses, only ban ones where you see the same ip address causing the same error on a regular basis. Note that sometimes you will see viewstate errors associated with ScriptResource.axd, some of these are definitely not normal traffic (in my log) but viewstate errors can also be caused by normal traffic if the user posts the form before the page has finished loading so I would not ban those unless its happening repeatedly from a particular ip address.