happened me the same several times in the champions league qualifying phase. the first leg is being played before the official squad registration date (screenshot - i can't confirm my squad registration and need to play with fake players :-) ) for that leg. I think this is not tied only to europa league but a kind of a squad registering bug. I have a save game, if you need it.
Europa League First Qualifying Round - Cannot Register Players
in Europe Competition Specific Issues
Posted
happened me the same several times in the champions league qualifying phase. the first leg is being played before the official squad registration date (screenshot - i can't confirm my squad registration and need to play with fake players :-) ) for that leg. I think this is not tied only to europa league but a kind of a squad registering bug. I have a save game, if you need it.