Cannot set custom attributes for mailboxes

Hello,

I'm facing a really annoying issue. I finished deploying brand new setup of Exchange 2010, upgraded to SP2 and started checking it.

Everything works fine, except Custom Attributes.

When I'm trying to specify custom attribute for mailboxes, both in the Shell and in Console, I'm getting the following error:

An object class violation occurred.Active Directory operation failed on AD01.domain.local. This error is not retriable. Additional information: An attempt was made to modify an object to include an attribute that is not legal for its class.

I have searched all over the net for something, nothing was found.

I thought that maybe AD wasn't configured properly, so I ran the following commands:

needles to say, nothing helped.

Can anyone here assist me with this error?