I have been setting up a custom mod pack on Minecraft 1.20.1, and found that the error; Invalid Player Data occurs on world creation when using the mod Simply Swords[Fabric and Forge], I had previously found that other mods to add Minecraft dungeons armor and weapons caused this issue as well, I am not sure that these are also incompatible but have a suspicion they may be.

Can't reproduce. Try using the mod with XL Packets or similar.