I don’t think ReadWritePaths=/usr/local/lib/owncloud is the right answer and I am surprised that solved it. Nextcloud is configured to use a different directory for data:
'datadirectory' => '/home/user-data/owncloud',
I’m not sure about what appstoreenabled is doing in this case.
i’m still having this issue on v75. Without appstoreenabled => false, i am unable to login to nextcloud or connect calendars over caldav. i’m not sure if it’s also related, but i also lost the ability to connect my iOS device to the Exchange endpoint as well (it kept prompting for my password - just like the caldav issue above)
As I am writing this after you have posted an updated version, this comment is likely not of immediate use. However, for future reference:
On my install, there is a link from: /usr/local/lib/owncloud/config/config.php to /home/user-data/owncloud/config.php. Maybe this is why the ReadWritePaths=/usr/local/lib/owncloud permission was required?
Check the package changes first, especially PHP/nginx, before upgrading to v75. The Nextcloud error points to a permissions/config issue, while the Z-Push authentication failures could be related to the same PHP or backend changes. The oc_users_external UNIQUE constraint error is also worth investigating separately, since it suggests a duplicate entry in the Nextcloud database. I’d take a backup and check the logs around the time of the updates, then compare the affected file ownership/permissions and PHP version with the previous working state. I wouldn’t restore everything or upgrade Mail-in-a-Box yet until the cause is clearer.