Problem: When a user is editing a checked-out version of a Word document and leaves the cursor in an edited comment field or in the header/ footer editing area, the check-in fails with the following error: “This document cannot be checked in while the Web server copy is open. Close the Web server copy and try again.” This occurs even though no other user has the document open.
Resolution: Have the user place his/her cursor in the main body of the document. Save and check in normally.
Thanks to the following thread for this solution.