A critical error has occurred.
Failed to load viewstate. The control tree into which viewstate is being loaded must match the control tree that was used to save viewstate during the previous request. For example, when adding controls dynamically, the controls added during a post-back must match the type and position of the controls added during the initial request.

I installed the new Events module.  I imported the AVCalendar events.  I had to manually copy over the categories, locations etc from the calendar tables, but this was not difficult.

I am getting an occasional viewstate error though.  I cleared the cache, turned off the blowery compression module and I was still getting the errors.  To recreate, try editting an event, Save.  Now navigate to a different page, now come back to your Events page.  This doesn't happen every time, but it happens enough to make me think something is going on here.

 

I installed the latest version (4.3.5) and had this issue as well all over the place.  After disabling compression in IIS (right click on Web Sites, choose Properties and uncheck everything under the Service tab) it all went away.  I have always wished I could use compression, but every time I try it fails in IIS.