Jack,

There can be a number of reasons for this.  You could try deleting the Windows Credentials (Credential Manager in Windows) for the user and then trying Sage again.

If this doesn't work then there are a couple of other options.

Regards,

if this is only happening on one user we have seen this caused by:-

DNS lookup, having IPV6 turned has confused sage

The users windows profile is having issues, so removing the profile and/or creating a new one has solved this. 

I think it basically related to a trust issue between the user/pc and the domain.