I've experienced this on many servers. It's usually a server with some custom playertypes.
Personally I think it might have something to do with animation name string length. There's an issue w/this where the combined string length of an object's named animations will be too long, and some of the animations will not work. Afaik people in Blockland have been getting around this problem by assembling the shapes w/dsqs. The animations will work, but, I guess it causes network issues w/trying to send too much info in a single packet?
That really is just my theory.
I ran into this problem while trying to make an animated 4 slot counter(4*10*2 animations). I opted to simplify the animation names and reduce the number of them, instead of trying to do any tricks with dsqs.
Oh! I also encountered this when modifying default datablocks. I was trying to change the order of datablocks so that an add-on's 1x5 would show up between the default 1x4 and 1x6. But this created a consistent error spouting some nonsense about a missing m.dts