SNAP gives TIFFopen errors displaying JP2 images if they have long filenames

this is a limitation of windows. The path cannot be longer then 260 characters.
If you move the file into the root directory of C:\ then it you can probably open it, too.

There is a workaround to enable longer paths, but I never tried it:
How to Enable Long Path Support in Windows 10 (msftnext.com)

There is nothing we can do about.