Error when importing code from TFS

We use Team Foundation Server and we would like to migrate to GitHub.
I’m trying to import our code from our TFS Server 2010 with the entire commits history but it fails with the following error: “There was an error importing commits.”.

There isn’t any details on the error.
I tried several times without success. I don’t have any idea on how I could resolve this problem.

Thanks in advance for any suggestion.
Stephane