MinerJohn

This is happening on a friend's Realms server.  It has broken a previously-working ice farm

Downloading and optimizing worked for us.  Seems to have been stable ever since.  Fingers crossed!

Having this issue too on Win10.  Takes ages to connect, then crashes randomly.  Sometimes you can play for 10-15 minutes, other times it crashes almost immediately.  I'm currently stuck in a portal in the nether trying to get back to the overworld, but every time I go through the portal, I crash.

Seems several others are having this; possible duplicate reports at MC-100285 (mine own) and MC-100283

I can replicate this - rolled a backup of my singleplayer world back to pre 1 and zombies started spawning immediately.