In order for me to get it to work I had to replace this file:
libmupen64plus.so.2.0.0
in /opt/retropie/emulators/mupen64plus/test/
I replaced it with the same file that i got from a friend, it was from an older version of retropie, it worked (games still don’t run great, but they run). I was getting a segmentation fault every time I tried to run mupen64plus.
If you send me a message with your email, I can send you the same file.