TFS Check In Issue: The underlying connection was closed: A connection that was expected to be kept alive was closed by the server

I've recently started seeing the below error when attempting to check in code to TFS:

The underlying connection was closed: A connection that was expected to be kept alive was closed by the server.

The TFS service is hosted by our vendor. When I connect directly to the internet I'm able to check in code. However when I connect via our LAN (which means I either go through a proxy server, or can reconfigure my connection to bypass the proxy and instead go via the firewall) I receive the above error. Equally consultants from the vendor based on site who VPN from our LAN to our vendor's network don't see the issue.

This issue is not unique to me, but is common to anyone trying to check in to TFS from our network (without VPN).

Only check-ins seem to be affected. Checking out code, browsing the repository, viewing history, or accessing the portal all work as expected.

When I look at the HTTP traffic using Fiddler I can see a number of HTTP 401 errors from posts to the urls /tfs/myCompanyName/VersionControl/v1.0/upload.ashx, /tfs/myCompanyName/VersionControl/v3.0/repository.asmx and /tfs/myCompanyName/VersionControl/v1.0/repository.asmx.

To our knowledge there have been no changes by the vendor to their TFS or firewalls, and on our side no changes to the proxy or firewall; but this functionality worked 2 weeks ago, and has stopped working now, so something's changed...

Any thoughts on what this could be, or how we could go about troubleshooting?

Thanks in advance.