Wysiwyg 7.x-2.1 Fatal error on any edit page of notes

I can only guess what's happened here. It would be interesting to know what class name it tried to use, but that might be difficult if you're not used to debugging PHP.
Did you update the module manually or via the GUI?
Have you re-downloaded/-extracted Wysiwyg 2.1 to make sure no files were corrupted?
If you're using FTP, have you tried changing the file transfer mode between ASCII/Auto/Binary and re-uploading the files?

I've had cPanel miss some files when extracting archives using it. If you haven't already, delete the wysiwyg folder and extract the wyswyg 7.x-2.1 archive again, clear caches and try again.

Are you able to find which Class name it's trying to use? You might have to insert a try{...}catch(){} statement around the line in common.inc and print out any variable which might hold its name. (Don't forget to call die() afterwards to prevent further execution in a broken state.

I've received the same error message under the same conditions, upgrading WYSIWYG 2.0 to 2.1. In my case, I was using drush to upgrade the module.
The class name that entity_get_controller is trying to use is wysiwyg_profile.

If you use drush, you must clear the cache (drush cc all) afterwards.
It sounds like the class registry has not picked up that the class exists in wysiwyg.module. (If you can't see it on line 31 of wysiwyg.module, something's really gone wrong!)

Maybe Registry Rebuild can help here if things get really bad?
Just read the instructions carefully!

I got same issue:

Fatal error: Class name must be a valid object or a string in .../includes/common.inc on line 7679

This happens when I go to links like:

/admin/config/content/wysiwyg
/node/add/..... (adding any new content type)

uninstalling module and database and re-installing seems to solve the problem