I tried your hints, and I get the information on non-accepted certificates. After transferring my wife's account to my newly installed Opera Mail on my notebook, it seemed to work fine, after allowing the certifikate, it was possible to send mails, but they could not be retrieved on my notebook in Opera Mail, yet in Outlook I could receive it. So it was sent, but was not retrievable
Since I have wasted (too) many hours I will stop now to "reanimate" Opera Mail and switch to Outlook
Thanks for your help !!!