Like many of you, I've been having problems syncing bluetooth controllers (NES/SNES Online.) It seems like pairing always works, but after that syncing seems hit or miss. Just now I discovered a way that seems to work very reliably.
While the MiSTer is running, unplug the bluetooth dongle, plug it back in, and then immediately try to sync. This worked for me 6 times in a row (testing both a NES and SNES controller), after trying and failing to sync around 10 times.
Update:
Actually disregard... I tried this again today and it didn't really work. Guess I just got lucky or something?