I am not, by all means, an specialist, but I guess this happens because of Nginx. Have you checked https://docs.moodle.org/310/en/Nginx ? Are you able to try with apache?
Also, this isn't related with themes so you should check something like https://moodle.org/mod/forum/discuss.php?d=322457
Also, this isn't related with themes so you should check something like https://moodle.org/mod/forum/discuss.php?d=322457