NWBC Logon failure

Hi,

Sometimes when the users use NWBC 4.0 to access the SAP system they receive the following error:

If I flush the DNS with the command ipconfig /flushdns it works but everyday some users present this issue again. I found the following OSS note (1832862 - NWBC cannot connect to the portal) that talk about this issue but my connection is not with /irj/portal.

Someone knows why is this happening?

Thanks for the help,

Carlos