Material buildup under silicon cover

[Jman15x 4]

Any idea what is happening here or what could cause this to occur? I think the clear part is ultimaker PC because that is the only transparent filament I have, similarly the "white-ish" glob must be ultimaker ABS. 

[Smithy 1,144]

It happens for example when the model gets loose during a print and the print head moves the part around. The material is somehow blocked when leaving the tip of the nozzle and filament goes through the silicon cover into the head. So you had luck that you have seen is early, otherwise your whole head gets floated.

 

Always check the condition of the silicon cover, as soon you see that it is not ok anymore, like the right one on your picture, you should replace it.