3 possibilities I can think of:

-Dropping framerate sounds like the Out Of Memory problem, esp. at turn 500.  Did you try exiting/restarting Elemental, not just reloading?

Regarding the OOM, when I see framerate starting to stutter and immediately try to save the game almost every time crashes with the OOM error and the save doesn't complete.  The act of saving precipitates the OOM.  Your save was successful, which might suggest it's not an OOM type issue causing your problem.

-Turns not being advancable is usually a unit on auto-explore or who have been given a multi-turn destination, and then get blocked or otherwise can't complete that turns movement (boats are especially likely to suffer this).  If you can find the unit and unblock it, that fixes this problem.

-Some report success in ending turns by zooming out to the cloth map, something about some animation has to play out on the screen, or some such thing.

I'm having same issue - current patch installed.  Game will not move passed a certain turn (724 if that matters).   I can reload a few turns before hand and it stops once again on the same turn, never ending the turn or something.

I have the same problem at turn 347. If you load this game it is not possible to advance to the next turn. the frame rate goes down to perhaps 1 or 2 fps. version 1.07. No boat and I think it is related to moving into autmatic battles and that you have to observe them on the map if they do not go into the tactical mode. Memory leaks are a good guess, this is what the debug.err contains for most part.

This contains the debug.err and the savegame.

http://hotfile.com/dl/67178794/74ad445/debug_endturn.7z.html