Try to sync to yahoo calendar. This worked several days ago and still works on my wife's android.
I have:
1) cleared cache
2) reinstalled onecalendar
3) deleted yahoo account on onecalendar and readded
4) deleted yahoo account on onecalendar and readded using newly created yahoo app password
Note that there is no apparent error thrown when adding account, only when trying to sync
I have the same issue. Did you by any chance sign up for Yahoo Mail Plus? That was about the time my sync started to fail. Before that, OneCalendar had been working perfectly. I have since cancelled the Yahoo Mail Plus subscriptio, but for some reason Yahoo is making me wait until this month's subscription runs out. After that, I can do some more testing.
Found on Reddit:
We're very sorry app passwords are unavailable for Yahoo Mail accounts. While we upgrade our services, the feature has been temporarily removed. We don't have an ETA on when this will return but we encourage you to check back in the future for updates on the restoration of this feature. Rest assured, you can still access your mail from the Yahoo Mail app or via a web browser. We apologize for any inconvenience this may cause you.
