After breeding 2 turtles the egg laying turtle will begin to look for a place to lay the eggs and sometimes they will give up looking and freeze. I am able to unfreeze them by going into a nether portal and coming back and seems to kick start the turtles again. Sometimes it's only temporary and the freeze again and i have to go into the nether a few times before they finally unfreeze enough to find a suitable place to lay the eggs. (There could be other ways to unfreeze them but going into the neither and back is what worked for me)

All my turtles are in an enclosure around an island but it is their home they spawned from and they all successfully lay their eggs here. 

Once i was able to nudge a frozen turtle and it unfroze itself when i nudged it into a suitable place to lay it's eggs. 

And to clarify, when they freeze, they just stop moving all together. They won't do or go anywhere. I even waited a whole day cycle and they remain frozen. And it is only the egg laying turtle that freezes. They don't freeze every time but it happens a lot.