This usually means something is wrong with your anim bp, usually it means its trying to play an invalid or null animation.
This is dependent on the skeletal mesh’s physics asset. The default character’s use Epic’s physics asset, which is a little prone to doing that. A lot of marketplace content has bad physics asset setups which cause this, which will require you to fix it yourself unfortunately.
As you and others discovered, apparently if UE4 runs at a high frame rate (250+ fps) the built in networking in the engine breaks down. I don’t have a proper solution for this yet.