We have a working ifolder 3.8 server.
Windows XP workstations can login and iFolder syncs.
No Windows 7 users (same users as have working XP workstations) can sync.

Error on client is:

System.Web.Services.Protocols.SoapException: Server was unable to process request. ---> System.ArgumentNullException: Key cannot be null.
Parameter name: key
at System.Collections.Hashtable.ContainsKey(Object key)
at Simias.WebState..ctor(String DomainID)
at Simias.DomainServices.DomainAgent.Login(Uri host, String domainID, NetworkCredential networkCredential, Boolean calledRecursive)
at Simias.DomainServices.DomainAgent.Attach(String host, String user, String password)
at Simias.Web.SimiasService.ConnectToDomain(String UserName, String Password, String Host)
--- End of inner exception stack trace ---

Any ideas on a solution?