45°38'N-13°47'E
Deity
Ok, simple bug. Fix will be pushed to the C23C SVN shortly (check in an hour or so). The window used to compound current inflation over to calculate the equilibrium level didn't cope with being before the start of the current speed setting's inflation start turn. You'll just need the two line change in CvPlayer::getEquilibriumInflationCostModifier() for porting across.
From a first quick test, it looks like it's working. I'll do some more test tomorrow and will report back if I find problems (but I guess there shouldn't be any). Thank you Koshling!
