Q:

Can't enable Gmail calendar syncing

I'm trying to enable calendar syncing for my work Gmail account (in System Preferences > Internet Accounts). I already have Mail and Messages enabled on the account with no problems. When I try to enable Calendars, it spins for a while and then gives me an error:

A secure connection couldn't be established.Click "Continue" to continue creating the account. You can edit the account later in
  Calendar preferences if necessary.

If I do click Continue, though, it just does the same thing again, so I can never actually enable it.

UPDATE: we recently transferred our email accounts to a new custom domain (we use Google Apps for business, so we don't use @gmail.com), and I'm having this exact issue again. Some additional details:

If I try to enable Calendar syncing via Calendar.app (rather than System Preferences > Internet Accounts), I get a different, very generic error:

There was a problem setting up the account.An unknown error occurred.

Using the same exact account info and credentials, I can enable calendar syncing on my iPhone, but my account never appears in Calendar as a group (alongside iCloud, etc.) and therefore I don't see any of its events. So it's still failing, but not giving me an error message like it does on the Mac, for whatever reason.

A:

After a ton of trial and error, I finally got this working. I don't know what the heck the issue was, but it was a total mess. At one point, I was somehow getting reminders for events in Notification Center even though they weren't appearing on my calendar.
Anyway, I had to:

Turn off ALL calendar syncing, including for other accounts (iCloud, Facebook, etc.)
Completely delete the problematic account from System Preferences > Internet Accounts
Completely delete the contents of ~/Library/Calendars**
Log out and back in

Then I set up the problematic account again from scratch (in System Preferences > Internet Accounts) and enabled calendar syncing. This time, it worked like normal, and then I was able to reenable calendar syncing for all my other accounts.
I didn't have to change any settings or anything on Google's end (google.com/calendar).
** Note: I was only able to do this because I don't store any calendars locally. If you have local calendars, deleting the contents of this folder will delete all those events. Just a warning!

