I've found infcloud [0] to be decent.

It's a bit messy to set up but the config file is extensively documented and you generally only need to touch it in the initial setup.

[0]: https://www.inf-it.com/open-source/clients/infcloud/

For CalDAV: http://agendav.org/ is much prettier than infCloud, but actually has it's own php-backend and mysql database. A bit heavier and is not strictly only-CalDAV. Could be a viable solution though.

For CardDAV: https://github.com/owncloud/contacts could be used as a frontend, since it advertises it's talking to CardDAV-backend, but it's not actually deployable as a standalone interface. It's better than infCloud both architecture-wise and UI-wise, and could be the best opensource solution available, if you're willing to adapt this frontend to stand-alone usage (or integrate into whatever you need it for)

Nextcloud works for me