Game crashes on worlds updated from pre-1.17 as soon as player 2 joins

We have several worlds created more than a year ago.

• Some were updated during the 1.17 Y-axis extension.
• One world was created fresh from 1.17 and crashes, so, it is not related to 1.17 updates
• New created worlds created in August, September work fine
• Crashing worlds were working fine just after the Y-axis update so we deleted the backups
• After a recent update they crash when the 2nd player joins, consistently
• They work fine with only 1 player
• After the 2nd player joins, the world seems to start loading for P2, but
  • Some/All of the mob are visible
  • Land is black, and still loading
  • In about 5 seconds the game crashes
  • VERY CONSISENT on most upgraded worlds.
  • We have about 5 worlds. most were updated from pre1.17.  They all crash within 5 seconds of 2nd player joining