Mail app - add mailbox not possible

I reinstalled Nextcloud today and wanted to set up the mails in the Mail app.
Ubuntu 22.04
Nextcloud Hub 4 (26.0.2)
Mail 3.2.0
I noticed in the mail app that I can’t add mailbox folders.
Without these mailbox folders, the server can’t download and store mails or send mails either, so errors occur.

What am I doing wrong? How can I solve the problem?

Thank you very much!