I'm getting this same error message with the new 1.18 patch. I could easily download my latest backups in 1.17, but not in 1.18. Same error message as OP has attached.
I get this issue only when using a tunnel or connecting outside my LAN. If I use localhost, then everything loads up perfectly. But when I use my static IP or a tunnel (on the same PC where I have my server), then the game fails to load and I fall through an empty void. Same goes for my friends who try to connect to my server.
This is still a problem in 1.18.1. Having many item frames in view causes serious framerate drop.