"No sent mailbox configured" Error when sending a message

Expected behavior

Tell us what should happen
After filling in the complete form to create a new mail account and saving without errors and syncing the inbox from the new account succussfully, I expect the app to send E-Mails without an error, assuming the remote IMAP server is available. The sent mail should occur in the sent folder.

Actual behavior

Tell us what happens instead
Sending a message results in a 400 error response with the information: {"status":"fail","data":{"message":"No sent mailbox configured","type":"OCA\Mail\Exception\SentMailboxNotSetException"}} (to be seen in the developer tools of chromium in the network section). The mail is not sent and not stored in the configured folder (INBOX.sent)

Sidenote: while writing a 400 error occurrs when the app tries to save the draft (No drafts mailbox configured...).

Mail app

Mail app version: (see apps admin page, e.g. 0.5.3)
1.7.0

Mailserver or service: (e.g. Outlook, Yahoo, Gmail, Exchange,...)
Remote hosted webserver (by onyxhosting) with postfix

Server configuration

Operating system: (e.g. Debian 8)
Ubuntu 20.04 LTS

Web server: (e.g. Apache, Nginx,...)
Nginx 1.7

Database: (e.g. MariaDB, SQLite or PostgreSQL)
PostgreSQL 12.5

PHP version: (e.g. 7.0)
7.4

Nextcloud Version: (see admin page, e.g. 13.0.2)
20.0.2

Client configuration

Browser: (e.g. Firefox 48)
Chromium 87.0

Operating system: (e.g. Arch Linux)
Arch Linux