I've spoken too quick! The problem is now solved: a bit of waiting did the trick.

Today, I completed a 4 weeks in a row daily missions and received my reward as expected (4330 cristals, 87 supplies of each and 29 repair kits). I then entered a battle and all of a sudden, it is as if I didn't get my rewards :( And now I can't access my daily missions: nothing happens when I click the button! Restarting the game didn't help. Will I get back my reward? Is there something I can do to fix that? Thank you!