Office 365 Exchange sync fail

Hello

I have recently updated my iPhone 14 Pro Max to iOS18.0

suddenly after, the calendar froze every time I tried to add an appointment. It is still like that

Even though it was very frustrating, I wasn’t going to make a big deal out of it because I had just ordered the new iPhone 16 pro max. Once I completed setup (from iCloud, not phone to phone) I added my email accounts. Everything worked fine from different accounts imap, google etc. (also the calendar) until I added my office 365 exchange account. Everything is set to sync in settings but nothing works properly. Contacts only syncing partly (approximately only 500 out of 1550 contacts are synced). The calendar freezes every time, and mail is not fetching anything else but the headers. When I open an email it just keeps loading but no text ever appears.

On my iPhone 14 that also runs iOS 18.0 I have the same exchange account added but the error only appears in calendar described earlier.

sorry for my long post, but I thing I need to explain this as detailed as possible.

Please help 🙏