So #ArchLinux's current TLS stack has issues when connecting to C.IM over HTTPS. In what has become a pretty weird bug with some armwrestling between Mozilla and Cloudflare, the workaround was thankfully quite easy to implement:
https://kzimmermann.0x.no/articles/fixing_cim_certificate_arch.html
Tilde Mirror: https://tilde.town/~kzimmermann/articles/fixing_cim_certificate_arch.html
Hey, at least I got #tut to work again.