Total Commander

it disappears if i switch to another app and back. sometimes.

In this case, it's probably Total Commander itself which gets unloaded. Try allowing it too in the background, in addition to the plugin.

then they come up with silly behaviors like closing background apps within 10 seconds...

It gives you great battery life for apps which track you in the background (which mine don't do).

I found a Solution for the "... has already been closed" error.
In Apps in the Android Options "TotalCmd-LAN..." you can stop the App.
Next time TC will create a new Connection. Works fine with my Samsung S7