Forget all of your points about contacting vendor support for a moment, and examine the key piece of why cPanel questions (and the vast majority of other GUI control panel questions) are off-topic here. This is from the close reason that was (appropriately) chosen for your question:
Questions involving web hosting control panels are off-topic at Server Fault because they customize their systems such that normal administration tools and methods no longer apply...
Period. Full stop. Do not pass go, do not collect $100. cPanel messes up a server to such an extreme extent that standard sysadmin practices, tools, troubleshooting techniques no longer apply.
So why choose to exclude cPanel -- arguably the #1 control panel for hosts to install...
Just because a software is popular doesn't give it a free pass here. We're not excluding cPanel (see my below "As an aside" comment).
I understand - you asked a question, hoping for an answer. The question got closed, which hurts a bit. The fact remains that the community is very strongly against accepting cPanel questions, and has spoken time and time again on the topic, always with the same outcome.
So, where to go from here: not to beat a dead horse, but call up cPanel support. Honestly. You're paying them for their software and support, so take advantage of it. They'll be able to help you better than anyone here can.
As an aside, we very rarely close questions due to someone not contacting vendor support before coming here. Very rarely. Your question was not closed because you didn't go through cPanel support. It was closed because the mere presence of cPanel on a server makes troubleshooting this sort of thing difficult if not impossible to do without causing the entire cPanel house of cards to fall down around you.