A few weeks ago, Google detected “unusual activity” when I logged into my email from home on a new computer. I was asked to verify my account via a code sent to me via SMS. This resulted in DAVx5 throwing errors and no longer syncing - on a different device. It looked like the Google servers had deemed my home IP address as untrustworthy.
My DAVx5 connection string is: https://email@example.com/events/. So I tried entering that as an address into my browser. A login prompt appeared so I logged in with my email address and password. The attempt failed.
Hmm, is the service no longer running? Then I tried with another account that I had. It worked. Damn, my account is definitely blocked here.
For two weeks I logged into my email from the same IP address. Every time Google required verification via SMS. Boring. During this time DAVx5 was constantly failing.
Then today it let me in without verification. At last, my IP address was trusted as far as my Google account was concerned. And guess what, DAVx5 started working again. Hooray!
So for those of you out there who cannot get DAVx5 to work with your account - or it suddenly stops working - you just have to wait and persevere by logging in daily until the Google servers associate your IP address and your email account together so that other services can log in from there without being blocked.
Hope this helps someone.