Evan Stern
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I'm having this exact same issue (although I was not on Realms; I was on a whitelisted server).
Died in the nether right by my portal (inside it, actually) and after clicking "Respawn" nothing happened. The multiplayer menu is still showing me as logged in (i.e. the number of people logged in shows me there).
Same error code is produced as the OP posted.
Also as OP reports, single player is fine and restarting the game has no effect.
I don't know if this will solve anyone's problem but I have been having a very similar issue to this and found a solution.
Using the old vanilla launcher I was getting between 70 and 80FPS normally but as soon as I switched to the snapshot's launcher that dropped to about 10FPS. After doing a lot of Google-Fu I was able to nail down the issue. Here's a link to the solution as posted on the Tekkit forum (the solution is actually for the "Magic Launcher" and Tekkit launcher – which were both also causing the same issue for me)
http://forums.technicpack.net/threads/low-fps-in-tekkit.12688/#post-97541
Basically, on certain Nvidia GPUs/Drivers (and on certain laptops in particular) the old launcher was able to get minecraft to run on the correct GPU, however, the new launcher (and the Magic Launcher/Tekkit launcher) break down and use the onboard GPU.
I suggest checking out the above post and, if applicable, trying to manually set things up so that java processes are shipped off to the correct GPU. Doing so fixed my problems with the new launcher beautifully.
@Marcono1234
For me, as mentioned, I was on a whitelisted server rather than on Realms. I was not the host of the server so I was unable to access the playerdata folder and can't comment on whether or not host could have done so.
I clicked Respawn immediately upon death. After a few attempts without it responding I exited the game.
As a follow up to my original reply: After a little time, the other person who was logged in left the game (either he logged out or some error occurred, I can't be sure). When he tried to re-enter the game again he was unable to enter and was getting the same error I saw. The only way we were able to resolve this was to contact the host and have him restart the server. Since then, no issues.