After changing back to another format the PDF editor loads again. However, when saving changes, an error message pops up saying:
"Coding error detected, it must be fixed by a programmer: Improper use of the assignment class. Cannot load the grade item."
Then the screen is greyed out and it gets stuck with the message "Saving Changes" and an infinitely looping spinny thing again.
If I refresh the page or close and open again, any changes I made do get saved.