I just made the big upgrade from 1.9.3 to 1.9.12 to 2.0.3 and I thought flawlessly until I try to upload a file to a course. I get the "Invalid JSON string" error and just that, no other information. Everything in all of my teachers' courses is there. Then after more searching I find that I cannot navigate to "Plugins/Repositories/Manage repositories." When I try, I get a full page blank white screen. In fact the only thing I can click on (and get anything) in the Repositories folder is "Common repository settings." Can anyone give me any direction. I have searched everything on Moodle.org and the web and have come up empty. Thanks.
I figured out the problem on my own.....after sifting through more webserver logs I found the line that the php was stopping on.....which pointed me to an error in my config.php. I guess the people that always ask "did you check your logs" are always right Thanks !