Jura Impressa F50 - Water is running beside it

Hello together,

with my Jura Impressa F50 the following error occurs after about every third
cup:

The coffee is ground, the water pump starts, but then
the water does not come out in the normal way, but lands
in the collecting tray. In the marc container then the
unused coffee powder ends up.

At the next cup it works again most of the time...

Maybe someone knows what could be going on?
A friend of mine is generally not technically clumsy and
could help me with a smaller mistake.

Many thanks and greetings !!

I once took a picture where exactly the water comes out:

http://www.imgimg.de/bild_F50v2f7f5a314JPG.jpg.html

The drainage valve's obviously jammed, and I'd replace it. It costs about 10 euros.