Update: To my knowledge without changing anything, I developed a different problem: I can now start a campaign. But I started one as Skaven, got the historical note and assassin dialogue (declined). Then I got an hourglass that lasted for a good long while, and finally an unspecified error and exit:
19:04:20.790 [game.script] [always] Germanicu5 ReallyBadAI v5.7 Running. Please upload log and savegames when reporting battle CTDs for SS.
19:06:55.275 [system.rpt] [error] Medieval 2: Total War encountered an unspecified error and will now exit.
As always I would welcome any suggestions.