Thanks a lot for your help.
If fact, I have set the time to 2 hours. So this would not be the problem.
I tried also to set the Enable cross app login, but it doesn't work.
After a successful login, if they press on search (the only button there), they are redirected to the login page again!
And only on iPhone.
This is really frustrating.
Thank you again for your help. Do you have another suggestion?