I'm getting the exact same issue as @asset using 4.0.2. I tried setting this up on the same server a year ago and had the same issue. I've tried different ways of installing (Softaculous, official installer, and so on), I've tried increasing the memory limit in php.ini because a memory handling error was being thrown for the dashboard widgets, I've tried different domains and subdomains under the same hosting package, and more. I've even tried creating /lib/max/Dal/Admin/Campaigns.php because it was missing but somehow that would always delete itself. (Quite astounded by that one actually - cPanel showed the file as existing when blank but adding the contents saw it delete itself, although the file seems not to exist in the original installation zip so I haven't tried too hard. I'm probably not understanding something fundamental there. Edit: it does exist.)
Only the Inventory>Campaigns page completely fails, while the dashboard widgets take turns in showing 500 errors (which means Campaigns does sometimes show up there) with little consistency.
Completely at a loss as to how to fix this and would really appreciate some help. I'm presently using Revive on another server and it's great, but I need to install it on this one.
Thank you anyone who can help for doing so!
Update: I cloned the other install over and it works just fine. No idea why, but new installs just don't work!
Update 2: Nope! I changed the header image and suddenly I have the same problem again.
Update 3: Looks like Campaigns.php is gone again. Any ideas what's deleting it?
Update 4: Even uploading the file under a different name is seeing it deleted immediately. I'm starting to wonder if this server has some kind of antivirus which is incorrectly detecting something dodgy in the file.