I have this issue now. It has previously affected another player on the same realm I play on. For him it happened after leaving the home screen on for some time, though this is not the case for me now. I just launched the game not having played for a couple of days and wen't directly to the realms menu. I have MacOS and he has PC. I was playing on the Realm without any issues when it was affecting him. He did not find a clear way of fix it, but tried restarting the game, restarting the computer, selecting different installations (optifine, plain vanilla, etc.), reinstalling the game, disabling java autoupdates and so on. For him realms was working fine again within an hour, but it is unclear if it was his doing.
Launching on a duplicate installation does not work for me.