So recently I’ve encountered the error in the subject. This came after 2 major changes (OS upgrade and PHP upgrade). So definitely it had to do with one of them. My money were on the first one. I’ve looked around on the internet and the only thing I could see was this old article that had no solution: https://www.digitalocean.com/community/questions/do-agent-exited-with-a-return-code-100

ctrl+alt+end

I’ve stumbled upon this great list of materials you can use to get started on IT, and Networking in particular. I have to admit that some are new for me, even after being in this field for over 12 years.