Error updating public store with busy information
If you decline the invitation (or do not receive one), subscribe to the calendar in your NOTE When user adds a shared calendar, they can apply custom properties (for example, colors, names, description) which does not affect the properties of the calendar owner.This behavior is contrary to Delegation, where any calendar property changes performed by the delegate directly affects the owner's calendar.You can also use an advanced type of sharing — delegation.
The error they receive from the sceduling assitant is "No free/busy information could be retreived" and the calendar is dashed out in light grey.
A 503 error message can be customized by the website it appears on, or the server software that generates it, so the ways in which you might see it vary 503 Service Unavailable errors can appear in any browser in any operating system, including Windows 10 back through Windows XP, mac OS, Linux, etc....
even your smartphone or other nontraditional computers.
The 503 Service Unavailable error is an HTTP status code that means the web site's server is simply not available right now.
Most of the time, it occurs because the server is too busy or because there's maintenance being performed on it. See the section further down the page for some things to look at if you're not sure what to do.