thanks for the hint. Indeed the JP2000 driver was missing on the machine where sen2cor 2.2.1 was installed.
I removed sen2cor and installed GDAL individually. JPEG2000 driver is there and .jp2 can be read (gdalinfo). Unfortunately installing sen2cor 2.2.1 overwrites the former gdal installation thus removing the driver. I tried to remove the gdal installation from the setup.py file but wasn’t successfull.
Any hints are welcome