If you are using Deathwynd's traits mod and you have activated more than 5 of the old traits, You can start a game but saves will not load. This game can't be fixed and you will have to delete it or just forget about it, but you can prevent future problems by editing your custom race traits per above.

I had the same problem and this worked for me. There could be other things causing a similar problem, I guess

I would try to load the game and then check out the debug.err file in the GalCiv III folder in the My Documents directory.  Often times the debug.err file will tell you what the game is choking on.

Tried to start a game with this mod freshly installed and got the following after the start up screen, but before the main menu appeared:

MOD VALIDATION

ERROR: Tech Tree Audit: Tech Tree Fracture detected.  This tech has no valid prerequisite, and is unreachable.

Tech Name: AreceanStartingTech

Tree: Arcean_Tree

Pretty sure I have it correctly installed.  If it is choking on the Tech Tree, that would/could explain why those entires in the debug.err file.  Not sure why you're not seeing that error when you start your game though.

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EDIT:::: The tech in question, on my system at least:

<!-- Begin Tech -->

<Tech>
<InternalName>ArceanStartingTech</InternalName>
<GenericName>ArceanStartingTech</GenericName>
<DisplayName>ArceanStartingTech_Name</DisplayName>
<TechTree>Arcean_Tree</TechTree>
<ShortDescription>ArceanStartingTech_ShortDec</ShortDescription>
<Description>ArceanStartingTech_Dec</Description>
<ColorDef>TechBlue</ColorDef>
<Icon>GC3_Engineering_Tech_Icon.png</Icon>
<ResearchCost>0</ResearchCost>
<AICategoryWeight>
<Military>1</Military>
<Growth>1</Growth>
<Tech>1</Tech>
<Diplomacy>1</Diplomacy>
<Expansion>1</Expansion>
<Wealth>1</Wealth>
<Influence>1</Influence>
<Fortification>1</Fortification>
</AICategoryWeight>
</Tech>

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Probably should take any more of this disucssion to the Acrean mod thread.  I'll go ahead and repost this there.