I had this problem before late afternoon and i solve it somehow but i bought my FC3 on Steam before î switched to standalone DCS launcher .
After i uninstalled and repair multiples time FC3 without succès i tried to unbound my Steam account and the licence on thé dcs internet webpage and rebound them . And in place of FC3 licence they have changed it to F-15C,SU-27 , MiG-29 etc and in the New launcher i could reinstall them .
I think it's all about thé licence.
I Hope it will help