What version of QNAP are you using? QNAP 4.3.3 does not have the module authz_groupfile_module loaded, so that's may be what is causing your Internal Server Error message. (AuthGroupFile /dev/null in .htaccess).
I think you can fix this by adding
LoadModule authz_groupfile_module modules/mod_authz_groupfile.so
to file: /mnt/HDA_ROOT/.config/apache/extra/apache-default-modules.conf
Or possibly just by leaving the AuthGroupFile /dev/null commented out.
I found this quote "Remove the line authgroupfile in your .htaccess if it's set to null as they have bodged the mod_authz_groupfile not loading by default in the 4.3.3 firmware"
Guide for enabling apache logs on QNAP:
https://technedigitale.com/archives/407