Multiple MMU2S Problems
While my MK3S itself runs perfectly fine without the MMU2S, the MMU2S has three separate issues.
1. It often doesn't feed the filament during loading.
The extruder starts turning, waiting for the filament to arrive, but the MMU2S just doesn't feed the filament at all. It's not entirely clear what happens, but either the idler isn't moved into place correctly, or the drive gear isn't turning. Sometimes (but not always), all lights are blinking when that happens. What's weird is that there's about a 20% probability that it works correctly, and when it does, the whole behavior of the MMU2S unit seems to be completely different: it homes the two motors that can be homed, and then feeds the filament correctly.
2. It jams during a print.
After about 15 hours of printing, the extruder suddenly stops extruding. Unscrewing the PTFE tube while the printer is still printing reveals the issue: the MMU2S unit is suddenly locking the filament in place, causing the extruder to grind through it. I'm not sure why this happens, but one possible explanation is that the motor that moves the idlers is slowly pulled by the moving filament until it catches the filament, and jams it.
3. It often doesn't unload
Even though it's easily possible to manually unload the filament, the MMU2S unit often doesn't manage to unload it.
I've already completely rebuilt the MMU2S unit with the newest STLs. Previous to the rebuild, I've also had the problem that the unit would constantly load and unload the filament (i.e. it would push it to the extruder, the extruder would catch it, but then it would error out and pull it back, then push it again, etc).
There are so many seemingly different problems that I'm not sure where to start, and since I've just rebuilt the whole thing a few days ago, I'm pretty sure there's no mechanical problem. Mechanically, the MMU2S unit appears to work just fine, everything moves okay, and sometimes loading works perfectly, but then it just stops working again for no obvious reason.
I have no idea where to even start troubleshooting this.
For lack of feed, I would first check to be sure those spring-loaded screws are properly adjusted.
I installed those long knobs on mine.
You might have to play around with testing, as in loading to extruder and unloading over and over and adjusting to see what tension works best for you.
Also any sharp bends in the tubes or any points of friction for the filament.
A good sanity check would be to inspect the MMU2S gears to be sure that no slippery filament residue has built up on them.
I cleaned everything in the MMU unit when rebuilding it, so I think that's okay. I also realigned the drive gears so they're perfectly aligned with the PTFE tubes. For the spring-loaded tension screws, I tried every position from very loose to super tight. If it's very loose, that does fix problem 2, the MMU no longer jams during the print, but it also means it can't properly push the filament to the extruder.
I had similar problems and I discover that the blade was slightly out of place. From time to time, the filament would touch the blade and stack. I remove it complete and I had prints with hundredths of changes with no problem.
Also had some major problems with loading and unloading.
Fixed the loading problem by moving the MMU back 3cm ( https://www.prusaprinters.org/prints/30254-mmu2s-holder-extended)
Fixed the unloading problem by changing the PTFE tube in the extruder because it was damaged by constantly trying to pry out filament and adding a PTFE locking clip to hold the tube in place ( https://www.thingiverse.com/thing:3559874)
Haven't had a loading/unloading problem since !!!
Mind you I have only used my MMU2S in single colour mode because I mostly do functional prints, but plan on dabbling into multi colour prints once my workplace has been organized.
After trying out a bunch of things, it seems to be working more reliably now. The thing that made the biggest difference was replacing the power and signal cables.