Cannot login from iOS mobile app

Hello,

I have a user who cannot access Appian from the iOS mobile app. He correctly enters the server url and  proceeds to next page, but after inserting username and password he is presented with the following error:

"Cannot complete activity: This activity cannot be completed while offline. Please try again when an internet or server connection is regained"

The user however is not offline. I also encounter no problems when logging in with his credentials from my iOS phone. Can anyone help me with this problem? Thanks

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