The tag {Marker:} does not appear on multiplayer, though it does single player

What command(s) do you used ?

Please force a crash by pressing F3 + C for 10 seconds while in-game and attach the crash report ([minecraft/crash-reports/crash-<DATE>-client.txt|http://hopper.minecraft.net/help/finding-minecraft-data-folder]) here.

I have used, /summon, and /entitydata

I will provide the crash log

Crash report (Debug Crash)

Is the server 1.8 or 1.8.1 ?

1.8, I take it that if it was 1.8.1, that the server would know to send the proper data of {Marker:}. Correct?

Sorry, I didn't look to see that it was released in the 1.8.1-pre. I think that was the problem. thanks though. I'll see if it works

Yes, the Marker tag was introduced in 1.8.1.

Yes it seems that was the issue, sorry for the rash goings on.