Tip on MMU2s with shell on side -block while retracting  

Tip on MMU2s with shell on side -block while retracting

My filament tips are quite ok but often there is a sort of "shell" forming on the side behind the tip.

this shell is sometimes loose and breaks off. this loose shell can get stuck at the Festo couplers and prevent the unloading process- then the filament gets stuck at the festo couplers.

Sometimes the loose "shell" messes the printer up at another place,because it just blocks things.

This gets usually just an issue after a few layers.

How can i get rid of this "shell" forming, while still keeping good tips?

(the tip in the photo is not my best one)

cooling moves will not improve the situation, i changed the nozzle to another 0.4mm one and temperature changes will alos not fix this problem but create other ones. I do not use prusament, but tried it and had the same problems.

MMU2s plus MK2.5