iCloud and iCal sync issue - 403 Error

I'm having a huge sync issue between my iCal and iCloud calendars. I've tried a number of fixes suggested on Apple Discussions, but none has worked. It's hit or miss... but only happens when I am adding events I was invited to - doesn't matter what calendar I'm assigning it to.

The server responded with an error.

Access to “xxx” in “xxx” in account “iCloud” is not permitted.

The server responded:

“403”

to operation CalDAVWriteEntityQueueableOperation.

We use invites at work for all calendar events and it's causing me a lot of frustration.

I've also just discovered that these same invites are showing up on the iCloud calendar when using my browser, but NOT on my iCloud synced iCal on my Mac. Online, they show that I've replied with "Accepted", but for some reason, they WILL NOT sync with my laptop... but they DO seem to show up on my iPhone.

I've seen this question asked a number of times on Apple Discussions, but no one seems to have come up with an answer, so I thought I'd ask it again.