I have the same problem, I only use the backend for editing and ading events. The old version 3.5.47 works, but the new do not.
Edit:
The strangest thing. I have been destroying a copy of the site without luck, after everything was removed it suddenly worked. Back to start, restore new copy site, did a few adjustments, and it worked. So back to live site, try to recreate steps, and suddenly it worked. I have RSFirewall installed and have protected my user from changes, so I removed protection on user, edited user without actually doing any changes, just enter user and save user. Editing and creating events works again. It also workes after reenableing protection for user.
I do not know what does this, but could be a user setting that was missing for the new version that was set when saving the user.
- espen