Microsoft Exchange – OU picker is empty when creating new user

We've migrated one of our Clients fairly early but recently we got complain that when new user is created from ECP on Exchange 2013 there's no way to pick Organizational Unit (OU) where the user is supposed to be.

It seems it's well known issue for Exchange Server 2013. It's described in KB303817 but a title is a bit misleading Exchange Server 2013 doesn't display all OUs when it creates a new mailbox. In our case it was not showing all organizational units and not just some of them. The fix is simple from there.

Check OU count in Exchange Management Shell

(Get-OrganizationalUnit -ResultSize unlimited).count

Modify each servers web.config: \\<server>\c$\Program Files\Microsoft\Exchange Server\V15\ClientAccess\ecp\web.config by adding

<!-- allows the OU picker when placing a new mailbox in its designated organizational unit to retrieve all OUs - default value is 500 -->
<add key="GetListDefaultResultSize" value="2000" />

just before AppSettings end </appSetting>

After that it's just matter of opening IIS, finding MSExchangeECPAppPool and recyling it. 

Do this for every server that holds Mailbox Role.