Thanks for the hint, looks like it was hitting some limits:
https://paste.debian.net/hidden/c1c46d21/ (~/.xsession)
I ran `xterm', `firefox-esr' and `emacs elfeed' together to make it crash. It says "Fatal IO error 35 (Resource temporarily unavailable) on X server". I wasn't able to replicate the `telnet' issue on this run.
I'm able to set those options in Caja.