I have some users for whom the Availability feature does not work properly. They are able to compose an email and include the chosen available meeting times they want to offer to the recipient(s).  However, when the recipient clicks on one of the time slots available to them to reserve that meeting time, the click renders a new browser window that shows the "Checking Availability" animation sequence for a short time, but then the recipient sees a message stating

"There was an error with your request. Please try again later."

What is the cause of this error message?  Is this likely a permissions setting that needs to be adjusted?  A profile setting? Security setting?

Any help would be appreciated!