I use the HTTPS CDN to accelerate, but the warning "because the WebDAV interface seems to be destroyed, so your web server does not have the correct settings to allow file synchronization."
a CDN is made for basic webpages and not for advanced file sharing solutions like ownCloud. Please disable this CDN for ownCloud as you will see other issues like explained here:
If you still have issues you can work your way through this FAQ:
If you have followed the advises above and still seeing this issue create a new thread in the "oC server" category and make sure that you're filling out the template shown when creating a new thread.