Boxer (iOS) problems to connect to Exchange 2016 – Connection refused


I got a strange problem with the SEG, Exchange 2016 and Boxer. Basically, a user can`t login in Boxer. The Boxer applications logs shows that the connection is refused by the exchange server.

“Response to device is (status code 400, status message Connection refused) …”

But this is very strange because there is another EMM Platform and a bunch of other devices which works perfectly with the same account and with EAS. I tried other email accounts too.
Of course I tried different possibilities for the username (UPN, E-Mail …) but nothing works. There`s no policy that blocks the device (in W1 or in Exchange).

Any ideas?