We've had hundreds of uploads since we moved to 5.x.

I just tried to upload a 5mb png file and got this error.

Sorry, an unknown server error occurred when uploading this file.

(Error code: -200)

Thoughts on why?

It uploaded fine here.

You would need to check your server error logs. Thats a generic error that is essentially telling you that your server said no, for whatever reason. Your error logs should tell you more