Wow, that is strange.
First of all, can you login and navigate to Events > Settings > Template – try change the Default Event Template setting and see if you can clear things up that way.
Failing that, though I note you tried deactivating other plugins, can you also try switching to a default, unmodified theme such as Twenty Eleven or Twenty Twelve? If that solves the problem we will at least know that the issue lies with your theme and can take things from there 🙂
Thanks!