This is a Moodle TeX filter display issue, NOT a dragmath issue. There are any number of possibilities that are addressed in the forum. Of course, the initial process, as it seems you have already started, is to make sure the Tex filter is turned on, turn on debugging, and work through ALL the debugging info to be obtained from texdebug.php (it looks like you have presented us with only one aspect of the data available.) In situations like these where the perms appear to be correct it could be group owner issues, php directive issues, etc.
HOWEVER..... Are you specifically relying on the Moodle TeX filter for some reason? If not, I would suggest that you avoid the hassles of the TeX filter (which does need some work) and instead use MathJax CDN via AdditionalHTML.
I suppose SOMEONE should have developed a key for debugging the Moodle TeX filter version by version and put that in the docs. Perhaps you will volunteer to put that together once you have your second instance up and running