If on a shared server, hard to believe one has full control. ;)
Ran across the same thing when assisting another user with same issues.
Find the mod_security log to see the rule that was triggered. It's a number. Then add a rule to mod_security that ignores only '/course/modedit.php'.
There is a very old posting in which a user posted a response to the issue.
https://moodle.org/mod/forum/discuss.php?d=95086
'SoS', Ken