With the Nextcloud Zimlet you can add files from Nextcloud to emails as attachments, links and inline images. You can also store emails and attachments from Zimbra to Nextcloud.
This Zimlet is in the official repos and can be installed via:
yum install zimbra-zimlet-nextcloud
apt install zimbra-zimlet-nextcloud
See: https://zimbra.github.io/documentation/zimbra-10/adminguide.html#_setting_up_nextcloud
Check your bruteforce protection settings in Nextcloud and try a truncate table bruteforce_attempts.
Nextcloud extension moved to it's own repo at: https://github.com/Zimbra/zm-nextcloud-extension