Cannot join a world in web version of minecraft classic (https://classic.minecraft.net/)

I can create my own world and it works great, but when my friend tries to join via link (or I try to do it from a different browser), the page only shows white screen (and nothing appears on the 'host' side (can't see anyone joining within the world)).

After the world is closed, the link opens fine and shows corresponding message (i.e. can't join the world, it's closed now)

Repro is straightforward:
1. Open https://classic.minecraft.net/

2. Copy invite link

3. Type username & enter the world (from the 'host' side)

4. Open the link in the other browser

Tried switching steps 3/4, didn't help. Tested with chrome & arc.

Attaching console output, seems like there's nothing suspicious:

Let me know if you need other details)