An unknown error occurred all the time When creating, editing or deleting anything in CP the notification with "An unknown error occurred" is appended at the top. 

I check the console and I get a 500 on these: 

/index.php/admin/actions/tasks/runPendingTasks
/index.php/admin/actions/tasks/getRunningTaskInfo


Nothing is in the logs that I can see.
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