I had the same issue.
I made a custom Map (of the earth 300 millions year in the past with the Hercinian mountain chain along the equator). I made some little changes and optimizations and at a certain point it didn't work anymore. (didn't work = when using this map for a game the game cannot load the next turn of the game...) I started analyzing the issue... One topic I'm still uncertain about map creation is the empty territories (or undefined territories). So I stated to test on the poles of my amazing Map!
CAUSE: At the top border of your map (south and north poles) when you do not put any territory the map is working fine. But when the empty territory goes over too much land tiles you can find this issue.
SOLUTION: Reduce the empty territory tiles on land. I could not figure out the critical mass because testing it requires always launching a new game. This is boring...
The Map Validator don't check this issue. Could be improved...
I hope it helps! It solve my issue.
I made a custom map that ive spent a few hours making & whenever I try to play it, it wont load past turn one. I will do the first turn & then click end turn and then says turn pending without ever going to turn 2.