I tried loading a saved game where I continued playing after the 600 turns limit on Endless, at turn 615. This is the error I get:

The operation was canceled.

  at Amplitude.Mercury.Game.Game+<DoStart>d__4.MoveNext () [0x00157] in <e606c76f24df4f658f27082af4ec6286>:0 

  at Amplitude.Coroutine.Run () [0x00019] in <a38bac599c844a99a8a4fcca3b37292e>:0 

I also tried loading other saves as well. The ones after turn 600 produced the same error, while I was able to load those before turn 600.