Viewstate errors can happen by things that manipulate the page before postback, it can also happen if the user clicks a button before the page is fully loaded. There are lots of things crawling the web that can cause errors to be logged, it is not something to be too concerned about unless actual users report they are having problems with your site.
In this specific case, if you google for Webster Pro V3.4 you will find that is is some kind of web browser control used in application development, it is not a typical web browser used by your average web visitor.
Hope it helps,
Joe