Users from external access are getting Outlook Credentials Prompt

Hi DevCentral,

I deployed iApp Exchange 2010 for a customer of us. Theyre using their own Exchange Server 2010. It was running fine for month. Now, out of nowhere, some Users are receiving the famous "Credentials Prompt" of Windows Security in Outlook.

The iApp is pretty default. The only changes while roll-out were: In VPE - SSO Credential Mapping is set to custom "mcget session.logon.last.logonname" instead of "session.logon.last.username". Also I went to the Access Profile and changed from the default "ntlm_sso" to HTTP-Basic authentication. Also in Connectivity -> Exchange als services are configured with out HTTP-Basic Profile.

For context: Its public services - in Town Hall is the Exchange Server (internal), and the Fire Department (external) is working over the Exchange in the Town Hall. Users from external (Fire Department) are getting in irregular time intervals like 15 min sometimes 5 min sometimes 30 min, the prompt. So it happens only from external access. Users in Fire Department are centralized and using a Thin Client with virtualized Desktop via Citrix.

If you need any other informations let me know. Its really annoying for our customers.