Quarantined after syncing new Exchange Account

On my X10ii,, I already have a working Exchange Account with mail and calendar.

A new Exchange Account however, will not sync. For another company, I started a trial period on Office365.com. So I am the Global Admin over there. I tried different ways:

The account will set up OK, but as soon as the syncing starts, the device gets Quarantined.

Microsoft staff have now tried three times in an online session to send commands through PowerShell to Exchange. At certain point, the device will show up as Allowed. All barriers seem to have been removed on the admin settings side, but still: after a sync, it gets Quarantined.

Does anybody have experience with this? Tips?