I've been struggling with this issue as well and after a couple of hours going through all the suggestions, I found my problem and I'm posting here in case this might help someone else.
I've been using formMule for a while and I, luckily, never ran into the problem before now.
Turns out that the sheet for which you want to run formMule on has to be the first sheet in the spreadsheet. In my investigations, I had unhid Template 1 making it the first sheet in the file and it was causing havoc! I could get into FormMule but it kept spinning and spinning creating more and more Template sheets for each time I canceled and restarted formMule. I finally saw the Status column being added to the Template sheet and realized what was happening. As soon as I deleted all the Template 1 sheets and moved the sheet I wanted to run against to the first position in the file, it worked !
Hope this helps someone else.