I had a similar issue a month ago. I could not add nor delete mount points.
I’m guessing the problem might be with a kernel upgrade.
It looks like a permissions issue somewhere.
I changed the permissions on /etc/fstab (owned by root) from 644 to 666 and was then successful updating the mount points. I’ll see if I can isolate where the edit is performed to see how max2play modifies the file.
Maybe the modification is not elevating privileges properly?
Edit: The change I mentioned above is not a proper fix due to security concerns.
Edit2: I checked and this page runs under user www-data and does not appear to request a privilege elevation.