Hi @graphix
I’m not 100% what the solution is here, but it is clear that some debugging will be need to find the culprit.
I would attempt removing all other plugins / theme to rule out a conflict with another plugin.
I would also try with another user account to rule out user role issues
Also, try turning DEBUG MODE on / off to see errors / fix PHP isues which could be breaking the async-upload.php file
Good luck mate
Cheers
E