It is now fixed with the 1.20 update, but thanks for trying to help.
The mod I was trying to add when it started was Unity Anti-Cheat 2.7.
The mods work fine in a world but not in my realm.
Its every mod I put in to my realm, not just one mod.
Yes REALMS-10655 does describe my issue.
It is now fixed with the 1.20 update, but thanks for trying to help.