Load/Unload issues - some tips that helped me
I battled with the MMU for the better part of 4 days having to fix load/unload issues almost after every tool change. I tried all sorts of things (such as messing with the tension screws, cleaning the hobbed pulleys, and dis/reassembling parts of the MMU) but I think the three most important things were:
DOUBLE CHECK ALL OF YOUR PTFE LINES
Make sure that the filament can go through smoothly. You should be able to push it through holding the filament lightly. If you have to pinch it and push hard there's something that can be improved. For me I found that I had tightened the plate from section 6, step 15 too tightly: https://help.prusa3d.com/en/guide/6-electronics-and-mmu2s-unit-assembly_44728#45328. This caused feeding line through the MMU to be just a bit too hard and so loading and unloading constantly got bungled. Also make sure that your spools are nicely positioned and roll easily and that none of your filament is kinked.
CHECK THE FINDA POSITION IN THE SELECTOR
A bunch of my failures seemed to be a case where the filament would be unloaded perpetually.... It would be pulled out of the extruder, through the selector, across the pulleys and back into the supply PTFE lines but the pulley motor would never stop. I messed around with jiggling the steel ball in the selector and that seemed to work (stop the unloading) a couple of times. Then I checked the LCD > Support > Sensor Info > FINDA and wondered if maybe it wasn't going from 1 to N/A or 0 quickly enough. I re-read this: https://help.prusa3d.com/en/guide/5-pulley-body-assembly_43152#44360 and decided to try adjusting the FINDA while printing with filament loaded. I turned/pulled the FINDA up a bit making sure to keep the reading in "Sensor Info" at 1. I ultimately ended with the metal part of the FINDA 15.9mm exposed above the printed part.
MAKE SURE YOUR FILAMENT TEMPS ARE CORRECT
If you use the default "Generic PLA", etc. it may not match the manufacturer recommended temperature. In a couple of cases I had filament where the temp was too high or too low by default and that resulted various issues when pulling the filament out of the extruder. Take the time to configure your filaments in PrusaSlicer so that they have the right temps. Doing this seemed to stop leaving "strings" or having the filament end be "knobby".
Hope this helps someone.