Repetitive error message on Calendar

Hi,

I'm having issues with my calendar. The error only applies to my Mac Calendar, I use the same synced calendar on my iphone and ipad and they work fine.

Basically every time I open Calendar on the Mac I get an error box pop up as hopefully shown in the pic. If I click 'Try Again' or 'OK' the box instantly reappears, making the calendar application completely unusable.

How can I get rid of this? I have several email accounts linked to my calendar. The gmail one I assume it's referring to was a work address which is no longer valid, but I can't get into the application to remove the link to this account! As I said both ipad and iphone run the same calendar perfectly.

Help! Please!