Help! Trouble with my calendar and icloud.

I am having trouble with my calendar and icloud.

Customers send me meeting invites and I can accept them no problem. But if they subsequently change the date or time I get an error message:

Access to “appointment name” in “Home” in account “iCloud” is not permitted.

The server responded:

“403”

to operation CalDAVWriteEntityQueueableOperation

sometimes the time changes in my calendar, sometimes it doesn't, and my customers keep getting messages back from icloud 'accepting' the original date, not the changed date!!

It is driving me an my clients crazy! any help would be appreciated.