1.12.2,1.12.2 with the latest forge, and 1.1.3 pre release 5 all give the same error message. I tried all three, and while the first two immediately gave me the above error, 1.13 pre 5 had to try and download first (as I have no snapshot worlds yet) but still gave the same error message after getting about 90% done downloading.
I just had this problem in Java 1.13.1. First they were freezing my game, now I can't ride them. I was spawning them from eggs, in a group of four (since there's four people on the server). Hope this gets fixed soon or that pat of my Halloween plan is kaput!