Edgardo, it could be permissions issue as well, it may will take an hour of yours, but follow tutorial and see if you have setup your server accordingly, you can view tutorial HERE.
IIS Log does not show anything apart from which page was being accessed and client information. and you havent mentioned file size as well.
Edgardo, what is the file size of your course?
500 errors on IIS gives out plenty of information as well, can you paste that here as well?
also, refer to my earlier post, you may have to increase value in request filtering and time outs, you can see post HERE.
Sure you can upgrade to 3.8.4, that is why I gave you choice of doing either.
go into server variables, and there you will be able to make changes to the requirements as required in environment, make sure you make changes in both session and global, restart MariaDB and see if changes are intact, if not, try enforcing them through my.cnf file.
also make a copy of database, and using HeidiSQL, you will be able to change the table file format to barracuda well, but do this after changing other parameters.
Just one SSD? split into three, one for O/S, Moodle and Database
if you intend to keep Database on same box, then set it up properly to make use of NUMA nodes, as database accessing memory will get performance hit as it will be scattered across 4 banks groups.