It's "E:\Civ4\Beyond the Sword\Mods\Community Civ V\" ...
I guess, Civ was actually only doing what it was told by its CivilizationIV.ini ... somehow the map (script) name must have got in there, what IMHO should not happen with public/private maps from mod (sub)folders.
I got rid of the error by starting a new BtS game on the Archipelago map, which turned the entry into:
[GAME]
; Map Script file name
Map = Archipelago
again, and everything is fine since then. Chalk this off as a Civ4 bug, i guess ...