Loop in extruder
What material are you using? flexible filament? if so make sure you have as little pressure on the bondtech gears as you can get away with but still make the filament feed properly.
If you have too high pressure on the filament it will get compressed and you most likely will end up with filament tangled up around the extruder stepper motor shaft and in some cases you may have to take your extruder apart to clear it properly.
Also note that flexible material plus mmu is a bad idea and will cause problems.