Can't modify existing appointment

[Proposal Manager]

I have scheduled several meetings recently using Outlook to send invitations.
However, when I try to save the meeting and send an update notice to the
added attendees, I get this error message:

"This item could not be saved because it has been changed by another
person..."

This has never happened before, and as far as I can tell, no one has been
changing the appointments on my calendar (why is it even possible for someone
else to change my appointments?)

Outlook then tells me to make a copy, cut and past changes from the "copy
of" version, etc., etc. all of which is confusing and doesn't seem to work.

What could be causing this?

Thanks,

Proposal Manager