I went through a couple of weeks of hell with a site I manage when we moved from site manager to cpanel. Whenever I was working in the back end, I was getting an authorization error. Finally I found a level one tech who said there was a known issue with mod security and the joomla back end.
During the process of troubleshooting, I gave access to the level 2 support for troubleshooting... and ended up with a broken site.. I figured I'd simply ask the level one guys to restore from a backup.. Well, that broke the ownership of the folders.
After a dozen or so frustrating, wasted hours, I deleted everything in public HTML and restored from the backup from the old site manager site. I suppose they got the mod security exception working because that site is working now.
So.. I am moving my second site from site manager to cpanel.. And guess what.. I'm getting locked out of the backend.. I initiated a ticket and they want access to the joomla backend so they can replicate what I've already been told is a known issue. It would be like if you gave someone the keys to your shiny new car and they brought it back covered with mud, a blown engine and severe body damage and now their asking for the keys to another shiny new car.
My dilemma is that I have about a dozen clients on site manager/joomla 2/5. I would like to keep them with westhost but if I have to go through this every time (The aggravation and the risk) it isn't worth it. I would consider moving them to one of the other hosting companies.
If you do a search in the WH forums for joomla mod security in the WH 4 forum, you will find that other people have had the same problem.