I helped myself now by saving the whole config, uninstalling AquaMail and then reinstall it on my mobile and restore the configuration. In the last step I added the new cert again. It seem the old cert in certmanager was somehow broken, at least I was unable to remove it manually from Aqua Mails cert store. The new cert could be removed easily. I tried before uninstalling the app.
However it should not be a problem if a cert becomes invalid, so that you have to reinstall Aqua Mail to get it running again.
Thanks
Rainer