This is so puzzling to me. I have no idea why it's trying to update your _versions table with a ClassName field. Is anyone else experiencing this error?
tzeller, can you do me a huge favor and try to set up a fresh install of SS with the event_calendar and nothing else, and tell me if you're able to get it working? I have a strong feeling the module is not compatible with something you're running. If it works, slowly add in more of your other code until you get the error again and hopefully we can pin it down.
Thanks for your testing. These are the things I really want to get ironed out. I wish I could duplicate the error, but I'm not able to get it.