iPhone app doesn't work anymore

[Andrex]

Since 2 days ago the app on 3 iPhone stopped working.
I've deleted and reinstalled it, but as soon as I hit the ok after I've inserted the server data, it closes and from that moment on it closes as soon as I open it. In the fraction of second I can see a page there there is a button with "try the demo server" or something similar.
Is anybody else having this problem? Any workaround?
Thanks!