Viva la bronze has issues unloading
I recently upgraded both MK3S+ to MK4 and MMU2S to MMU3. The printer works well with no errors (at least none that were the printers fault) with the exception of my viva la bronze PLA filament. So far I printed approximately 5-10 color prints with some 2000 filament changes and no error. Unfortunately viva la bronze was the main filament for my first print after the upgrade after the sheep. The nextruder unloads the filament well, but the MMU is not able to pull it out of the nextruder afterwards. Pulling it manually is quite easy, no excessive force is required. I tried using it both in position number 3 and 4, both give the same issue but print any other filament well. The filament could be wet, but then all of my filaments are probably wet as most of them are the same age and some are used more often than viva la bornze. So far after the printer upgrade I used 6 different PLA and 2 different PETG filaments and only viva la bronze PLA is giving me any problems.
RE: Viva la bronze has issues unloading
I had a similar issue with my MK4 + MMU3. I printed a test sheep fine, but then ran into "Finda Filam. Stuck" errors at unload, when I tried using other filaments. I was using Prusament filaments (both PLA & PETG), all that had been heated in a Sunlu S4. It was driving me crazy because I'd have to restart the printer and then could run the 'unload filament' command and it'd work just fine. I also noticed that the tips of the filament were often stringy after they had been unloaded.
After a LOT of messages with support, we eventually found that lowering the nozzle temp by 5 degrees worked. Good luck!
For others running into the same issue, make sure to also check your idler screws tension, run your calibrations, and do some cold pulls until the filament comes out clean.