Format programs are not compatible between these releases. You need to reset it (the reset button) in order for formatting to work.
There is no reset button. The error message says *the custom format program has been reset" which seems to imply that it has *already been reset*. As I explained, the problem is only on the initial load of SQL Developer after upgrading; I don't get this error on subsequent starts.
Either way, I didn't have any custom format program so I'm not sure why this would be applicable. I would class this as a bug in SQL Developer.