I think it may be conflicting sub-mods. I have installed Mith's new EDU Beta, the two diplomacy mods ("locked alliances" and "stronger alliances"), Agostino's additional units mod, the "nerfed garrison script", reworked names mod, proofreading mod, slower ageing generals mod and "no faction borders tweak". All meticulously to their instructions.
Lol. That's most of the sub-mods in there. My campaign returns to the main menu when I select a campaign and click "start". Custom battles work fine. Why?