Sorry! Of Course:
My Server is running Openmediavault (Debian). Nextcloud is running in a Docker Container, the Nextcloud Version is 184.108.40.206, the Version of the Mail App is 0.17.0.
I use the Mail Client to have Acces to two Mail Accounts, one at Google Mail, the other one at Strato. I can read the Mails without problems. Furthermore I can reply an incoming mail. But when I try to write a new Mail and I type in the mail Adress and then I click out of the box, the mail Adress is gone. I guess, that I normaly have to click on a apearing checkbox to confirm the Mail adress, but the Mail Adresses I type in doest’t appear in that checkbox. Take a look at my Screenshot.