Does this happen with all plugins deactivated? Have you tried using a default theme? (2013/2014)
Now that I’ve finally got that site stable I’m nervous about messing with it any more. I tried editing a different site, which uses many of the same plugins but a different theme (“Commune”), and there didn’t seem to be a problem.
No new plugins had been added since I was last editing panels without any problem, and I apply all updates as they crop up to WordPress and plugins. Do you think it’s a problem with a plugin?