Solution 1:
Fixed it with rm -rf /etc/apt/trusted.gpg.d/*
Apparently, gpg of some old repository was messing things up, despite those repositories being disabled..
Update from 14.04 to 16.04 failed with GPG error
Solution 1:
Fixed it with rm -rf /etc/apt/trusted.gpg.d/*
Apparently, gpg of some old repository was messing things up, despite those repositories being disabled..