is there any possibility that it's not Simple YouTube to MP3/MP4 Converter alone that's to blame, but some kind of combination of it and another extension?
And is it a good idea for me to post a SAF topic which warns about the add-on?
Yes, that is always possible. Many vendors do inadequate testing of their programs, and on top of that it would be impossible for them to test their product cooperating with a plethora of various other software out there.
Sure, you can post your experience; it may be that someone else is having the same problem, does a search and comes up with your post. Glad you found the problem!