Hi Gerhard,
To narrow down the issue, I'd like to collect the following information:
1. About “Outlook 2015” you mentioned, I’d like to confirm the version of the “outlook 2015”. As far as I know, current Outlook for Mac 15.6 we can use, or do you mean Office for Mac 2015?
2. You mentioned “uninstalled outlook 2011”, do you mean before that, emails could be synced to Outlook 2011 and it worked fine?
3. Which account is set up on Outlook, an Office 365 Exchange Online account or another account like Outlook.com?
4. Could you test another account on your Outlook 2015 to see whether the same issue persists?
If you have any updates, please feel free to let me know.
Best regards,
Jamie