Excuse us for the late response. We don't offer the possibility to restart your sandbox solutions as of today. We have been discussing whether to provide such an option and if there is a significant amount of requests for that we will do our best to provide it in the near future.
It is possible to receive such kind of messages if you are preloading the page much faster as the viewstate field is not yet loaded, and thus cannot be properly validated on the server. With the 4.1 release that is due next week, we have made significant improvements of the lifecycle of our pages which should get rid of such problems.
the Telerik team