Dropping a portal to a new instance can destroy the current world instance permanently since a few days. The timer of the portal then doesn't decrease and players can't walk into it. Players joining that old instance won't get past the Initialization page and stay in a black sphere. There is already a canny describing the issue, but that one didn't describe what is actually causing the issue (https://feedback.vrchat.com/bug-reports/p/cannot-get-past-initializing-world-page)

This happens so often that it's not hard to reproduce.

In our tests, we did this in SDK3 worlds and the portal dropper also created the current instance and was master in there. We stayed a significant time in the instance before dropping the portal. Several users tried to join into that broken instance and they were all stuck and had to Alt+F4

Here is my logfile from the moment I joined one of the broken world instances: https://pastebin.com/eBYF3Uix

We can reproduce this issue when needed.