I don't know if this duplicates or relates to MC-103601, I searched for "ticking player" on Mojira's open bugposts, but couldn't find the same, only other bugposts containing "ticking", sorry if I overlooked an open bugpost for "player".
I did find a closed/fixed bugpost for "ticking player" from 2013 though: MC-35653
How it happened was: I opened up one of my bugtest worlds in 17w17a and upon entering immediately crashed with that error message in my client output; crash log file attached.
When I entered the world again (in 17w17a), everything was fine though, I didn't crash again.
I'll see if I can maybe reproduce it again, until then I'll leave out the "b"-version as affected, as it happened in "a". Maybe the crashlog can provide some information what was happening.
Are you sure it's the same? It is indeed my EnderDragon testworld where I enter directly into The End, but the Dragon is already in there (I summon them via command, not the crystals by the way, for some of my bugposts), and I still can use the world now? In MC-116656 it says the world would be unusable afterwards?
I'll keep an eye open though regarding the Dragon, it's possible it happens during the check whether the Dragon in there is the original/first Dragon or a new one 🙂