Can't send Emails

I’m not able to send emails.
The roblem is, that I can’t put in a Mail Adress in the send to box. When I write a Mail Adress here “” and I click in another box after that, the mail Adress disapears.

Is there some kind of trick in using the Mail client?

Thank You!!!

would you mind giving out a bit more informations about your setup, environment, config?

Sorry! Of Course:
My Server is running Openmediavault (Debian). Nextcloud is running in a Docker Container, the Nextcloud Version is, 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.