Check the error_log for the domain in question. There may be more clues.
Also, we recommend you use SymlinkIfOwnerMatch rather than FollowSymlinks (there are some significant security implications to Follow Symlinks, particularly if you have a shared system).
chown -h mydomain.com:mydomain.com /home/mydomain.com/public_html/blah/blah/blah/marks-video.mp4 ; ## notice -h flag in chown command
and that worked. apparently the symbolic link ownership matters. no changes at all to either (dot)htaccess or httpd.conf files.
one would think i would have known this for all the decades i have been using linux. every day i am amazed to learn just how much i really dont know about practically everything.