Some VPN software implement hacks that allow select apps to connect to their service while letting other apps connect to the internet without VPN. Unfortunately, some of these does not work with Minecraft and would make Minecraft run into this issue.
If this is the case, try turning off your VPN.
Zombie pigman or new mob?
Major changes introduced to the game necessitated this prompt. More precisely I'll call this "Major conversion required to use this world"
The text has been significantly revised from what it was when the bug was reported.
Please replace "Cannot Reproduce" with "Fixed"