The "crossed-out globe" icon appears and disappears randomly. Don't really know which app or state it's representing. Seems like an unavailable connection.
What does it mean?
Thanks to user Firelord, I went to System to check which version of Android and CalyxOS I do have. I stumbled over the "Statusbar icons" area. There you can enable/disable notification icons. The crossed-out globe stays for Firewall. I assume, that a firewall match (block) shows this icon.