This error is usually (often? always?) generated if someone tries to use an earlier version of the Editor (3 versions were released, one for each successive version of Civ3*) to load a ruleset file-type generated by a later version (there are 3 ruleset-formats, .bic, .bix and .biq: again, one for each successive version of the game/ Editor). The names and default-locations of 3 Civ3 Editor .exe files are (from memory): .../[Civilization III]/Civilization3Edit.exe => opens and saves only the .bic format (for use with 'vanilla' Civ3 v.1.29) .../[Civilization III]/Civ3PTW/Civilization3XEdit.exe => can open .bic and .bix formats, but saves only in the .bix format (for use with PTW v.1.27) .../[Civilization III]/Conquests/Civ3ConqestsEdit.exe => can open .bic, .bix, and .biq formats, but saves only in the .biq format (for use with Conquests/Complete v.1.22) (The GamersGate and GOG version supply all 3 Editor-versions — I don't know about the Steam version.) So my question would be, are you sure you were using the right Editor to re-open your .biq file? Spoiler * : Just to add another wrinkle, there were 2 versions of the Conquests Editor (v.1.00, and v.1.03); the latter was released with the final patch of the conquests.exe (v.1.22) — but managed to break the 'Scientific Leaders' option Although .biq files generated in the v.1.00 Conquests-Editor should be forwards-compatible with v.1.03, I wouldn't swear that that was the case. So if your mod was an old file, that might also be a possible cause of this error-message/problem.