Hope this may be the correct area to post this - it's a bit of a mad one!
Two of our users (a staff member and pupil) are getting the following error when signing in to portal.office.com, OneDrive, or SharePoint on their iPad's browser...
AADSTS165000: Invalid Request: (The user session context is missing. One or more of the user context values (cookies; form fields; headers) were not supplied, every request must include these values and maintain them across a complete single user flow.
When I look at the sign-in logs against either user and find the sign-in failure, the error code I get is 399218.
This error appears to only happen on iPads (we've tested multiple iPads, too), and on both our Smoothwall Browser (based on Safari) and Google Chrome. No matter whether we clear the browser cache completely, fully wipe the iPad, or try private browsing, the error still persists.
Interestingly, when I sign in as a user that isn't having issues, then sign them out and sign the problematic user in, it works. I'm at a total loss as to what's causing the issue. There's no discernible difference between the working pupil account and the non-working account, either!