Since I was already updating the server, I though I’d install munin as well. I have used munin 20 years ago at the dormitory and I realized, why I liked it. 🙂 Anyway, while I was installing, and checking, I saw that apache started to max out the CPU. After experimenting with it for a while, it turned out that somehow it is related to HTTPs and mod_gnutls. It turned out that I was not the only one with this problem:
Debian Bug report logs – #942737
libapache2-mod-gnutls: mod_gnutls consumes 100% cpu
So one more thing in the server maintenance backlog…