An unknown error occurred all the time
I too just had this problem - upon any action in the control panel flashed "An unknown error occurred". 

What I found in my craft web.log was: 
"ErrorException: ignore_user_abort() has been disabled for security reasons in"

Apparently in my server setup via Runcloud there are various advanced boilerplate Disable_Functions. I removed "ignore_user_abort". Problem solved. 

Now the question is what does ignore_user_abort actually do... 

Craft 3
PHP 7.2