Does anyone of you have an idea how to fix this?
I cannot update packages via Plesk and trying to run "apt-get update" throws an error with invalid signature EXPKEYSIG. I searched on the internet but nothing really helped, it still occures.
From the screen shot, it's complaining about the signature for panopta, not plesk. In fact the warning even said something about emailing them so you're better option is to email panopta support for getting that fixed.