DAVx⁵ has been successfully tested with 1CRM.

1CRM is a self-hosted / cloud-hosted business management suite.

DAVx⁵ settings

You need two separate accounts for CalDAV and CardDAV. Use "Login with URL/user name".

CalDAV Base URL: https://yoursubdomain.1crmcloud.com/caldav.php/

CardDAV Base URL: https://yoursubdomain.1crmcloud.com/carddav.php/

Full-day events may be displayed differently on Android than in your 1CRM settings. Contact photos are not supported and can't be synchronized.