When I asked my wife what her thoughts were on how to solve a problem where everything is spinning she yold me to quit drinking beer and do something useful ;)
This is hard, especially the part where i actually start doing something...
...but this might just be what may have been the cause of your issue...
...ask your Google Domain administrator if (s)he has restricted the use of Google Sheets extensions?
This causes all kinds of problems which deny FormMule from working properly.
If you're using a private / personal/ non-domain Google account the start a private browsing session (ctrl+shift+N) and login into this Gooogle account, the open the sheet(s) again with FM configured to determine if FM works then as normal (or not)
If it does a (synchronised) issue with your cookies or misbehaving extension might be the root-cause.
Let us know as soon as you gave nore information;)
Cheers, Bart