Suddenly first layer problems (visible gaps, bad adhesion)  

Suddenly first layer problems (visible gaps, bad adhesion)

I've a MK4s since two months, until yesterday everything was fine (or nearly most of all). But since then i have problems with the first layer(s). Following image shows several clips (all where printed on the smooth PEI plate with Prusament PLA):

• the black ones on the left side were printed two days ago: the bottom layer is smooth.
• the white large ones on the left side where printed yesterday: the bottom layer has visible lines, but the surface is smooth (except if you touch it with the fingernails) - but not complete evenly.
• the white small ones on the right side where printend yesterday just after the small ones: there are visible gaps in the surface, which are palpable with the fingertips easily. The surface is very uneven.

As a startet printing two month ago, it was hardly possible to identify the lines, so even the quality of the bottom layer of black clips shown above is not as good as the bottom layer was after the assembly of the printer.

I've printend a first layer/bed level test:

Visible gaps, bad layer adhesion.

What i've found out so far, is that this could be an Z-Offset issue or underextrusion? The probing/auto leveling at the beginning of the print seems to look fine. I've run a calibration for the z-axis too, but the first layer test looks as in the image above.

All shown prints are printed with Prusament PLA and their default profiles from Prusa Slicer.

I would appreciate your help, cause i don't now where to start the troubleshooting, especially as the problem occured from one print to another.