I've been unable to have DavDroid access my Synology DSM Calendar using customized URL with alias as it can be set up in DSM settings as displayed here:
This allows to use "https://<fqdn>/calendar/" instead of "https://<fqdn>:<port>/"
That's not a really big deal but maybe you should investigate.
I had just published another post (which I've deleted since) thinking the Synology specific workaround didn't worked anymore, but further testing as convinced me that te problem is elsewhere.
Using the form <host>:<port> works perfectly so I'm Ok, although I have to open an extra port on my router which I would have liked to avoid.