Exchange 2010/Outlook 2013 user proposes new time for meeting - Other can't accept it - grayed out


We are running Exchange 2010 SP3.
The one user I will be refering to runs latest Office 2013 Pro.
Problem was happening in Office 2010 with that user.

I set up a meeting, that user proposes a new time, I am noticed that the meeting was cancelled and that the user proposes a new time, but I am unable to accept/decline the proposition, those buttons being grayed out.

The meeting shows in my agenda at the new proposed time, as I I had accepted it.
When I try to set up a test meeting with my fellow sysadmin, he can propose new time, and I am able to accept.
Seems to only be happenning with that account, with other people than me as well.

Any clue ?
I don't...
Been googling for a few days, nothing good came out...
This guy being a manager, I would love to correct this asap...

Thanks for your help, experts !
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maxalarieAuthor Commented:
And I don't have that "Auto-accept" thingy selected.
imkotteesSenior Messaging EngineerCommented:
Do you have access to his calendar? what is the permission set for anonymous and default users?

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maxalarieAuthor Commented:
Sorry for late reply, was sick yesterday and was locked out of the site this morning for billing/renewal issue.
I will check the permissions to his calendar, sounds like this might be the problem.

Thanks !
maxalarieAuthor Commented:
That did it, thanks !
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