Calendar was unable to save changes to event

Hi,

I have recently upgraded to High Sierra in my iMac (27-inch, Mid 2011). I now have an issue with my Calendar. I cannot add new events with a recipient - if I do I get the following error

Calendar was unable to save changes to event

There was an internal error attempting to save this event. The changes made were not saved.

I have tried clearing the cache files, but to no avail.

I run two primary calendars, and another side effect appears to be that when I get an invite to one account, it seems to put it in my calendar twice - one for each account?!

It is very strange and I cannot seem to find a way to fix it.

I also have a MacBook Air and an iPhone8 - neither of these seem to be affected (apart from obviously getting the duplicate events my iMac puts in!)

Any ideas please?

Thanks in advance.