A closer look into this points towards the plugin not identifying the metafile correctly, or alternatively constructing the metafile name in a wrong way when used with SNAP6.0. As shown in the screenshot in the other post, it passes the TCI.jp2 in the GRANULE[…]\IMG_DATA\ folder as metafile, whereas the correct would be the MTD*.xml in the top level of the *.SAFE folder.
Unfortunately I have no idea where/how to modify the plugin.
Instead, I tweaked sentinel2_metadata.py a little by adding this at the very beginning of the parse method:
if ((os.path.splitext(input_file)[1]!='MTD_MSIL1C.xml') or
(os.path.splitext(os.path.split(input_file)[0])[1]!='.SAFE')):
print('wrong metafile name detected. adapting.')
input_file = input_file.split('.SAFE')[0]+'.SAFE'+os.sep+'MTD_MSIL1C.xml'
print('parsing %s' %input_file)
Not 100% sure it works on every system and with any S2 data, but for me & my system & data, it did the trick.