Hey there Furbie1Clancy2,
We want to help make sure you're able to use your app as expected. You've done a great job troubleshooting this far. Have you checked for any app updates in the App Store? If so, you can also try reinstalling the app to test.
If you continue to run into complications we would recommend that you reach out to the app developer directly. They'll be best suited to help investigate. Especially since the issue started after updating your iPhone. The article below will help.
If an app unexpectedly quits, stops responding, or won’t open
Warm regards.