This happens every time. I have read all the threads I can find on this, tried the fixes, junk mail, etc etc etc and to be honest, the mods here are giving bad advice.

The problem:

"Sorry, this invitation isn't valid anymore. The sender may have taken away your permission, or you may have already declined this invitation. If you'd like to see this calendar, ask its owner to share it with you again."

My private Outlook gives permission to share, as soon as it arrives in my Office365 inbox I accept it and it FAILS!, tried different browsers.

Simple error, can I have a fix please without all the rubbish advice.

Awesome