There is an incompatibility with Immersive Portals 6.0.3 and Polymer Bundled 0.916, running under MC 1.21.1. Fabric API 0.105 is the only other mod installed. MC runs fine until loading a single player world, even a newly generated world, then the error message "Connection Lost. Invalid player data" appears and a button for Back to Server List despite trying to load a single player world. Removing either Immersive Portals or Polymer solves the problem.