I have Sharepoint 2010 Foundation running on Windows Server 2008 R2 Standard.
My problem is that 2 out of 4 users that we've tested is having problem with Checking in document from MS Office 2007/2010. I have enforced document versioning. All 4 users are under Contributor permission for the site and it's library.
If I checked out the document with "Use my local drafts folder", on a Windows 7 Ultimate with Office 2007 Standard SP2, the doc open fine.

But after editing, when I try to check in the document,

after the Check In form, I'll be prompted to authenticate 3 times (even after we checked Remember my credentials),

after that it will show "Word did not save the document".

If I save & close Word and do the check in from the web UI, after the Check In form, we'll get an error "This document was checked out to your local drafts folder but the local copy could not be checked in to the site."

If I checked out without using "Use my local drafts folder", the file becomes read only.
The only way is to upload the document manually, then it would work.
Another user who is having is problem is running on a Windows 7 Home Premium with Office 2010 Standard. It seems to be related to machine rather than user, as when I tried to create another user and login on my non-working machine, the same problem appears.
Any idea on the issue? I've tried googled around, only found problem alike, but no the same issue.