gpg bug Show more
There appears to be a bug in newer versions of gpg which basically means you can't authenticate (at least) some packages, including Nextcloud desktop.
If this command --
gpg --keyserver keyserver.ubuntu.com --recv-keys 1FCD77DD0DBEF5699AD2610160EE47FBAD3DD469
...results in "gpg: keyserver receive failed: Server indicated a failure", then you're seeing the bug.
More info: https://dev.gnupg.org/T3168
applies to Ubuntu 18.04 and Mint 19.1 (probably many other distros)