Don't know if this will help, but I had this exact problem.  Try this, it worked for me:

First, I installed libfuse2.  This won't remove libfuse3 but will give you compatibility with the older fuse libs which is sometimes a problem with some appimages.

and then type this into terminal:

$ ln -s /bin/fusermount3 /usr/bin

this should help with the permissions problem, among other things.