I'm having the same issue too, it's driving me mad. Every time I go into the app or website it asks me to update my contact details, and I've probably tried every possible combination of number and dialling code. Initially I was able to do seat selection, but now when I try to change my seats following an aircraft change, it tells me there's an error with my contact details and the changes can't be saved. I also have an issue that it's asking me to select my title from a list, but the list doesn't actually drop down so I can't select anything, which generates an error when I try to save. I'll try contacting customer services.