Exchange synch/profile issue

Dear community, I need help to solve a problem.
1.) I'm using Outlook classic 2016
2.) I need two mail accounts in outlook
a.) my gmail account - all good with that one
b.) my AWS Workmail account
3.) all used to work fine till I had another problem with teams integration into my calendar which I tried to fix without success. After a while I thought, that when I setup outlook from scratch that this could solve the problem - but it got worse.
4.) when I started the newly installed outlook, I can load my gmail account, and when I add my AWS Workmail account, I get the message that the account was successfully added and need to restart Outlook.
5.) After the restart I get the two messages:
- The name cannot be matched to a name in the address list.
- Cannot start Microsoft Outlook. Cannot open the Outlook window. The set of folders cannot be opened. An unexpected error has occurred.
6.) so the AWS Workmail is not loaded. I also tried to add the account manually in all different ways but without success. The integration of the AWS mail account on my Android Outlook works perfectly fine.

I dont know where the problem is and tried to get answers from AI and Microsoft Support without success. Anyone has an idea?

Many thanks, Rob