Mini developed print issues with first layer (Prusament PETG)  

Mini developed print issues with first layer (Prusament PETG)

After a couple of months of printing Prusament PLA and PETG beautifully, the printer now seems to have issues with the first layer. Yesterday I tried to start the same print several times, always failing to produce an acceptable first layer. It looks like the filament is "bubbling"? I tried several things:

• I tried black and orange PETG, same result but the issue is easier to see with orange PETG
• I use the textured PEI plate for PETG; tried cleaning it with IPA - no difference.
• I store all filament spools in the original ZIP bag if not in use, so moisture does not seem likely.
• In sharp corners, filament did not properly stick so I performed First Layer Calibration several times -  to no avail.
  I now get the feeling that the nozzle is too low, but if I set it higher, filament does not properly stick in sharp corners.
• I think the photos look like under-extrusion (EDIT: see last photo)? All slicer settings are at default (PrusaSlicer 2.3.0 RC2)

I don't know what to try next. Any suggestions on how to start troubleshooting this issue?