Hi Huig,
I have done some further research for the issue and found out that you’re integrating Office 365 with Windows Server 2012 Essentials (Sorry that I haven’t figured out that it’s the same with Windows Small Business server which can directly be integrated with Office 365 before).
However since our forum doesn’t focus on the server side settings part, to get the issue resolved more quickly, I suggest you post a new thread in our Windows Server 2012 Essentials Forum, thanks for your time.
Another workaround for the issue is that we can disable the integration in Office 365 Portal, which will stop sync between Office 365 and the local server, then you can edit the users directly in Office 365. We don’t recommend this way if you still want to use the dashboard to manage Office 365 accounts. You can find more details via: Disable Office 365 integration.