what happens here?  

RE:

This happened also to me now and then in the past, and it was caused by a nozzle partially encrusted with material.

Unless you screwed up the PrusaSlicer profile settings by playing with them, I think this happened also to you.

I think that at a certain point, some residues stays inside the nozzle, blocking the normal flow of the material. It happened also in my MK3S.

I tried to clean up the nozzle but there was only a temporary improvement, and then it failed one or two print later, so I had to replace the nozzle.

Now I know that when it happens I need to replace the nozzle.  The point is that on MK3S one nozzle costed around 3-7€, but now on MK4 is much more expansive ☹️ 

In another rare case, it depends from the too much force required to roll the spool (too much friction) and the filament is too tight.

Regards

RE: what happens here?

May I add a couple of more options: 
- Temperature creep. When the heatbreak is not seperating cold and hot enough the filament a bit above the nozzle can also melt and causing a problem there. 
- Heatbreak and PTFE tube not matching exactly (PTFE must have perfectly right shape see manuals). 
- Filament not dry enough. 
Hopefully not all at the same time 😉 

RE: what happens here?

I'd also like to suggest trying a different filament, too. I've had some cheap rolls of filament exhibit this behavior, occasionally, with my MK3 devices and now my MK4 devices. Turns out when I measure the filament with calipers, the width is extremely off, well outside of the claimed +-.02/.03/.05, etc.. that the cheapo brands claim. 

(I've actually speculated that some of the cheap, unheard of brands, are just rejected spools of filament from decent manufacturers.. for whatever reason.. which is why there's such a wide disparity between their rolls.... *shrug*)

Just another possibility, depending on where you source your filament from.