Someone on the Sonarr Git posted this. I had to remove a bunch of dependencies but was finally able to get things working again.
Pin your version of mono to 5.8.0.127 for the time being.
I did something like this (on Ubuntu):
apt remove mono-complete mono-devel (or whatever you installed for mono, I did mono-complete)
apt autoremove
I edited /etc/apt/sources.list.d/mono-xamarin.list to read:
deb http://download.mono-project.com/repo/ubuntu xenial/snapshots/5.8.0.127 main
Note in the above line, I locked to the 5.8.0.127 version, which still works with sonarr/radarr/jackett.
Then I did an apt update.
Then I did an apt install mono-complete, that got me back down to 5.8.0.127.
Hope this helps someone.
Most helpful comment
@Taloth is right, the problem is caused by NLog, apparently the nuget 5.0 beta doesn't contain the fix.
If you don't want to downgrade mono you can use the fixed version provided by @dutch2005: