Hi Boz Is it just the appointments or other activities as well, I have seen something similar in CRM 3.0 when a user does not have access to Queues in CRM i.e the security setting is set to no green blob for that user. Also something similar but not exactly the same when the security setting were throttled...

Have you checked Technet? There are a number of examples of this error message with reports. http://search.technet.microsoft.com/Default.aspx?locale=en-us&Query=%22An%20error%20has%20occurred.%20for%20more%20information%2c%20contact%20your%20system%20administrator.%22%20CRM%20report&lang=00&Brand...