In my system the Submit dataconnection is explicitly coded to the "friendly" hostname. If the form is opened in the Infopath client via a different URL, i.e. with an alias for the same host, the submit operation will always fail. The error message names the current user as the "other user" who is locking the file. Since both URLs work exactly the same in the browser, the subtle difference in the addressbar is easy to miss, even for experienced users. So this may appear to be an intermittent problem, depending upon the exact navigation path the user takes to reach the document library in a particular visit.
Perhaps the edits were propagated to the server version of the form by a Save operation rather than Submit? Even if Save is disabled in the menu and toolbar, closing the form with unsubmitted changes may prompt the user to save changes.
Note that Infopath operates somewhat differently with SharePoint than Word or Excel library documents. Having the form open, even without any edits or checkout, still locks the server version of the file. It is quite easy for a user to forget the file is open and inadvertently prevent anyone else from changing it.