Using the stack here's what happens:

1.) You cast fireball, declare your targets and the value of X

2.) Your opponent responds by putting an ability on the stack to give one creature shroud.

3.) FOLO, so the shroud giving ability resolves first.

4.) Fireball resolves and makes a check on each of its targets. Since one target is now illegal, it's excluded from receiving damage.

5.) You do X damage, divided evenly, to the one remaining target

608 Resolving Spells and Abilities

608.2b If the spell or ability specifies targets, it checks whether the targets are still legal. A target that’s no longer in the zone it was in when it was targeted is illegal. Other changes to the game state may cause a target to no longer be legal; for example, its characteristics may have changed or an effect may have changed the text of the spell. If the source of an ability has left the zone it was in, its last known information is used during this process. The spell or ability is countered if all its targets, for every instance of the word “target,” are now illegal. If the spell or ability is not countered, it will resolve normally, affecting only the targets that are still legal. If a target is illegal, the spell or ability can’t perform any actions on it or make the target perform any actions.