Some email apps will open the app in a web browser and not the app when you click a link in a notification email. When that happens you will see that warning as the app does not work in a web browser. Close the mail app and open the dating app. If this happens several times then your phone has cached the page. You should hard quite the app and clear the phone browser cache in phone settings settings.
Q. But I'm on my phone not a computer
A. Mobile phones use web browsers. Your iPhone uses Safari.
Q. I'm not using a web browser.
A. Most email apps (yahoo is a know issue) have a built in web browser. When you tap a normal link in an email it opens the site in it's own browser (maybe they like to know what you're looking at). Most times you'll not even realise it's a browser. Links to apps use a different type of link. Unfortunately not all email apps will read the special code and simply open it's built-in browser. Most apps will not work in a web browser which is why you'll see that warning page.
Q. But I opened the app and I still see that warning page.
A. Your phone will cache pages and load them locally to save you bandwidth and load times. If you have encountered the above problem then your phone browser cache will need emptying.From your iPhone screen - Setting - Safari - Clear History and website Data.