What is the problem with my ical on iMac OS ?

After downloading a calendar of events from a web site, I now get this message constantly:

Access to “Spring: Atlanta 2 - Miami 1” in “Jim's Calendar” in account “iCloud” is not permitted.

The server responded:

“403”

to operation CalDAVWriteEntityQueueableOperation.

Then I get two options: "Go Offline" or "Revert to Server " after clicking both of them several times, it goes away but comes back almost immediately.

How do I undo this??