Authentication issues in Outlook for iOS and/or Android

Hello 2 all,

I'm Administrator in a german Company. We are using Microsoft 365 Business Premium Licenses for our employes in a AD, AAD-hybrid environement with Passthru Authentication. We are using a Exchange Server 2016 with the newest CU installed behind a nginx reverse Proxy and modern Authentication for Login. Our problem is, that from time to time, several employes can not login to their email-accounts using the official outlook app from microsoft in iOS and/or android. Other apps are working and the employes can login with no issues, but not the official microsoft app. We tried every solution we could find anywhere on the internet, but none of them were successful. For some employees it was possible to reinstall the app, after which the login worked again, but others can't login till this day an there is no sulution to find. No failed Login attempt in the logs anywhere. We are desperate to find out what is wrong with Microsoft, that there are always so many problems with the services, and we simply cannot find a solution. the outlook app itself only says that in this moment the login isn't possible, and please try later.

Can anybody help us please? Monday this week, the number of affected employees increased again and there is no rule, why only this few amployes were affected this time or anytime..