It took a while, because there are a couple of things you simply "have to know":
- To communicate encrypted, the certificate must be trusted by your phone. If you have opted out of the CA-mafia and created your own, self-signed CA certificate, this means having to have a pin, pattern or password on your lock screen, and a constant warning about your network being monitored. Pointing the browser on your phone at your ca.crt file will import the certificate easily enough, though.
- You need to configure radiCALe to use your certificate and remember to turn on SSL.
- You must remember to use the correct type of htpasswd-file for authentication (if that is what you choose); it must match radiCALe's config.
- You must place your addressbook.vcf in /var/lib/radicale/collections/USERNAME/addressbook.vcf/addressbook.vcf - yes, it looks like a directory too much.
- You must place your calendar files in /var/lib/radicale/collections/USERNAME/calendar.ics/ - all of them. Otherwise DAVdroid/radiCALe will only show one of them. You can't place your other calendars in another subdir, they won't show up in DAVdroid.
- Use https://example.org:5232/USERNAME/ as the path to your server in DAVdroid.
- If the phone says there is a temporary syncronization problem with your contacts, try adding a new contact to the DAVdroid account. When I did that, sync pulled in all the contacts.
I am only sync'ing my web-based calender on the server in one direction (server to phone); this seems to just work. Addressbook will probably only be in the other direction (phone to server).
I managed to avoid sending my contacts to Google, by turning off all sync before creating the Google account (this is done in Settings/Data Usage top right menu "Auto-sync data"). After creating the account, I turned off wi-fi and mobile data - and then I reenabled Auto-sync data, and went to Settings/Accounts/Google and disabled sync for everything Google there. After that I could enable mobile data and wi-fi again.
The point here is for the email app to update Auto-sync data must be on, but if I turned it on again while there was a network connection available, my contacts would have been synced to the new Google account before I could disable syncing to it - thus the turn off wi-fi and mobile data "trick".
Update: If you try setting up a DAVdroid account, and you get an "HTTP error: 500 internal server error", while radiCALe reports no problem in the log, double check the permissions/ownership of the files in /var/lib/radicale/collections/ For some reason one of the .props-files got root.root as owner.group, changing to radicale.radicale fixed the problem.
Also, if DAVdoid says there is in intermittent problem with syncing your calendars, check that your .ics-files have UID entries for all VEVENTS - they are apparently mandatory for DAVdroid.