When I get calendar invites from a client (who has sent an invite to me from Lotus Notes in a government agency) on the mac, i accept them but later i get repeated messages saying "the server responded with an error... Access to <meeting> in "On My Ipad" in account "iCloud" is not permitted.
The server responded:
“403”
to operation CalDAVWriteEntityQueueableOperation.

I never had a problem before with these invites before upgrading to OSX Lion last week.