The problem isn’t that I can’t add masks, it’s that they’ve already been added and are now gone after deleting “unused masks” when I deleted the render files.
Restoring from a backup doesn’t work either.
No files have been moved and nothing else has been changed. Just that this happened when I deleted the render files and clicked on “delete unused magnetic masks”.
There doesn’t seem to be a solution for this so it looks like I’ll need to redo all the magnetic masks.