I had a user coming up to me saying that she couldn’t send calendar requests anymore. The error was: “Calendar sharing is not available with the following entries because of permission settings on your network”. As all mailboxes had just migrated to the Exchange 2010 server this could be anything. However a blog post from Nubby Admin saved me a lot of work.
The error was caused by an outdated entry in the user’s autocomplete list. After using the GAL to select the recipient all went well. I also instructed her to clear her autocomplete list.