Cannot edit existing content
I had an issue similar to this. The update to Drupal core caused issues with content types using the References Dialog module. We noticed that altering any nodes with a type using these reference dialog fields had issues. What tipped me off was that the other content types didn't have an issue. Odds are that this error is being caused by another module. Here are a couple things you can try in a development environment:


Create a new content type with core fields (a title, body, and maybe an image field will work). Do you get the same problem?
Verify whether or not all of your content types are affected. I know you said you can't alter any content, but you can save yourself time by being absolutely sure. Choose a few nodes for each type and try editing them to see if you get any other issues. If you see that there are only some content types affected, you can find out what fields / widgets they're using and start to deduct which one is causing the problem.


Good luck!