Ahhh! Running the .exe as an administrator directly from the install directory worked! (note: it did nothing when running the Epic launcher as admin). Thanks for the fast and working solution : -)
I have exactly the same issue. Got Gal civ 3 free on Epic, proceeded to buy every one of the DLC (apart from the soundtrack). Played for 3 hours yesterday and now today I cant even load my saves as it says I dont have the DLC installed. Everything re-installed, purged toggled in Epic and retoggled (to download again). Please fix this ASAP as I really wanted to play this game and if the DLC wont work Ill have to refund which I really dont want to do.