This is an issue that initially started when  I was using Minecraft Java Realms at about 1:00 AM on July 9th 2021. I downloaded a backup of my SMP world to modify a datapack and reuploaded it a bit later however I was unable to join because of "Invalid Player Data". I found that my Optifine version had recently become outdated as preview version 26 for 1.17 had just come out so I downloaded that but it came up with the same message. I then just switched to 1.17.1 but the same connection lost message appeared. I am able to access a backup I made using only Optifine 1.17 preview 26 however uploading it to Realms wont let me access it. This error doesnt apply to older worlds I have on Realms, only the newer backups of this one current world. I'm not sure if my friends can join this world or not.