DAVx⁵ has been successfully tested with Teambox.

Teambox is a Web-based ERP software product for the creative sector which can be configured individually.

DAVx⁵ settings

Base URL:

User name and password: your Teambox credentials

Calendars and tasks can be synchronized. Note that read-only task lists are not supported by OpenTasks, so make sure that you don't modify read-only task lists on your Android device.