To whom this may interest, the problem was first reported by Selur, 4 days ago,
http://forum.doom9.org/showthread.php?t=168437 :

https seems to be broken,..

just wanted to note that https://forum.doom9.org doesn't work atm.
Besides, the following Doom9 mirrors also have gone "offline" since then:

https://leffster.se/

https://nic.dnsalias.com/

https://www.koepi.info/

Wild guesses and conspiracy theories are welcome