Calender app refuses to show or sync first google account's calenders

So I've got an issue with the Calender app, possibly due to HTC's sense fuckery;

I got a new HTC One M8.

I added a primary Google account (A) during the phone setup, synced my apps my contacts and mail, all the jazz. Then I set up a secondary Google account (B), only syncing mail and Calender for that one, since it's just an old account with some lingering items.

However, today when I tried adding a Calender entry, it defaulted to B's calender. When I try to change to a different one I get:

• HTC Sync Manager / Outlook (Which I don't use)
• HTC TV Guide (I don't watch TV...)
• Account B's Calender.

When I go to the list of accounts and calenders to sync, it shows account A twice, and account B once.

Account B works correctly, but one entry for Account A is blank. The other entry for Account A shows my 3 calenders for that account, unticked.

When I tick these entries and go back, nothing changes. When I inspect the menu again, they are un-ticked again. After a few attemps to tick the calenders, both entries for A suddenly show no calenders or keep flickering between "no calenders" and unticked calenders.

I've tried:

• Rebooting
• Clearing the Calender Apps data
• Disabling calender sync for both accounts and re-enabling it just for account A.

None of it worked and it's driving me mental!

Between the constant nagging of HTC Sense to take over every bloody aspect of my Google account and the half-dozen other nag-issues with this phone, I'm considering just taking it back. Beautiful piece of kit, but it's caused me nothing but issues.