1
Settings

Go to the settings menu in your iOS device.

2
Safari

Select the Safari browser from the list.

3
Cross-Site-Tracking

Scroll to the Data and Privacy section and deactivate "prevent cross-site tracking". 

4
Clear Website Data

Scroll a bit further and click the "Clear History and Website Data" field

5
Close your Apps

Go to your task manager and close the Grape app and your Safari browser.

6
Open Grape

Now you should be all set. Restart the Grape App and log in with SSO.