Oh no! Happened to us in France, only a week ago. A known problem with the original fabric it appears. Unfortunately it's the fabric, as you rewind it, which folds the arms in. Best option is to use some rope to hold the arms, in against the tension of the arms, but another is to do the same but to remove the whole awning so it's safe to travel. It's two Torx T30 bolts next to each arm, above the sliding door, which allows you to then lift the whole cassette off. Bit wieldy to then travel with. Obviously, the best option is to affect a temporary repair to the fabric, reinstall it, so at least you can get the cassette to close. Sadly, it's an expensive bit of fabric to replace ~£300, but ours is currently being repaired by a sailmaker for about £60. It should be stronger than the original wield, so if it is a clean split/tear then do consider that.
Hope this helps
Steve