Your calendars are being synced with Microsoft Exchange over the air

Dear all!

iTunes on Windows tells me "Your calendars are being synced with Microsoft Exchange over the air" but I do not have any connection to Exchange since 2 years now (I left the company where I had the Exchange account two years ago).

Now I bought a Mac Mini and cannot sync my calendar from the iPhone to Mac OS. Mac OS also tells me that the iPhone is connected to MS Exchange, which is not the case.

Syncing my iPad´s calendars (with the same Apple ID logged in) to the Mac is no problem.

There is no Exchange account and no MDM on the iPhone anymore.

I do NOT use iCloud Calendar on any of my Apple devices.

Any ideas out there?

Kind regards,

Stephan