In most cases the cause for this error message is that authentication method configured in Allocatus Server and Exchange Server does not match. Please find out if the WebDAV virtual directory Exchange is configured for Basic, NTLM/Integrated Windows or Forms-based authentication and set the value accordingly on the Exchange Connection tab in Allocatus Server Configuration.
Often the Exchange Server indicates connection issues with HTTP Error 403 Forbidden. To find out the root cause paste the url which Allocatus uses to access the Exchange Server into the Internet Explorer's address bar and logon with the credentials of the Allocatus Service account. This will give you more detail. Please refer to following troubleshooting tips regarding this and other issues:
403 - Forbidden: Access is denied.
HTTP Error 403.6 - Forbidden: IP address of the client has been rejected.
Internet Information Services (IIS)
Contact your Exchange administrator and request allowance for the IP address of the Allocatus Server on the WebDAV directory
507 - Insufficient Space to Store Resource
The mailbox is full. Please contact the owner of the mailbox to clean up items.
Could not retrieve the distinguished name for Domain\UserName. Error: NotFound
The Windows account Domain\UserName is not existing in Active Directory. Remove the user from the users list in the Allocatus Configuration dialog or ask your administrator to create an Active Directory account for this user.
The primary email address of the user Domain\UserName could not be determined.
The Windows account is existing, but does not have an Exchange mailbox. Remove the user from the users list in the Allocatus Configuration dialog or ask your Exchange administrator to create a mailbox for the user.
The underlying connection was closed: An unexpected error occurred on a receive.
Most often this is caused by an invalid SSL certificate. Please make sure the Exchange Server has a valid SSL certificate with the exact hostname you configured in Allocatus Server and on the server running Allocatus Server is a root certificate of the issuing certificate authority available.