after respawning, you cannot move and this doesnt fix unless you re-log. clicking will cause [Server thread/WARN] [minecraft/MinecraftServer]: Player x tried to attack an invalid entity

I'm unable to reproduce this issue, but it's not out of the question that we're doing something improper with regards to teleporting the player around. User reports seem to indicate this only happens with Optifine installed, however. It could very well be an issue on their side.