iApp Exchange 2010 - iPhone users getting ERROR #401

Hello together!

Hope anybody can help me with that issue:

I deployed iApp Exchange 2010 for a customer. Everything works fine, but when users try to visit the OWA page like "outlook.customer.com" they will get a "401 - Not authorized (smth like this: Access denied because of invalid informations)"

Android Users are NOT facing that problem, they can login.

Device: iPhone 6, iOS 12.3.1

Does someone have an idea?

Blessings