No Write Access for Nextcloud Android App

Good Day Community,
I tried to use Nextcloud as a Backup Path for Threema.
I do get the following error message.
“Kein Schreibzugriff auf dieses Verzeichnis”
“No write access to this directory”
Already talked to the Threema Suppport who mentioned to get in touch with nextcloud.
Any ideas?


Here are the issues I see related to no write access.