Very happy to hear you’re finding it useful!
Here’s a few things to try:
(1) Apart from closing the document in Acrobat, perhaps try closing Acrobat itself.
(2) When exporting the PDF with FormMaker, check the “Show export dialog” checkbox. In the export dialog box, disable (temporarily) “View PDF after Exporting”. Does that work?
(3) Try exporting the file as an interactive PDF without using FormMaker. Does that work or does that also throw an error?
(4) Try restarting InDesign and Acrobat.
(5) Are you using OneDrive or Dropbox? I’ve noticed that sometimes if those apps are busy uploading files, those files are marked as “in use” and can’t be overwritten. Try temporarily pausing syncing in those apps, or disabling them.
(6) If all else files, restart your computer (there might be some instance of Acrobat running in the background, and restarting the computer will kill that).
I think one of those options should work. Let me know how it goes!