Nozzle touching/ digging print

[studiomarkwiechmann 0]

Hello,

 

during prints with a Ultimaker 2+ my nozzle keeps touching into the material,
digging it's nose into the freshly printed material, warping the whole print.

 

At the and of the prints the build plate doesn't seems to move down anymore,
finishing at a certain height, just spinning around, melting PLA with its tip.

 

Earlier prints with the same models didn't have the problem.

 

So the problem seems to be related to the Z axis,
calibration between prints doesn't't seems to help.

 

Is there something I'm missing?

 

Thanks in advance!