Reason: Invalid security certificate
Your server is using a security certificate that is not trusted by operating system of your device. It might be caused due to a misconfiguration or an attacker is intercepting your connection.
If you encounter such an error message, you should contact the owner of the server and inform them of the error, if possible. It is also recommended that you wait for the server to get fixed before using it. Unless you know and understand the technical reason- why the server presented incorrect identification, and are willing to risk communicating over a connection that could be vulnerable to an eavesdropper, you should not proceed.
If you are hosting your own personal WebDAV/ownCloud server using a self-signed certificate, please double check the certificate info and continue after trusting it.
Disable your WebDAV/ownCloud sync and re-enable it again.