Can't import widgets into v11.1 from a v9.0 widget export

When I export a widget from a v9.0 build it won't import into v11.1 - it just seems to hang on this screen:

Telligent.Evolution.Components.CSException: Widget Management: Could not export fragments ---> System.Threading.ThreadAbortException: Thread was being aborted.
 at System.Threading.Thread.AbortInternal()
 at System.Threading.Thread.Abort(Object stateInfo)
 at System.Web.HttpResponse.AbortCurrentThread()
 at System.Web.HttpResponse.End()
 at System.Web.HttpResponseWrapper.End()
 at Telligent.Evolution.Api.Plugins.Administration.ContentFragmentManagement.ContentFragmentManagementExportService.ExportFragments(IList`1 fragments, HttpContextBase context)
 at Telligent.Evolution.Api.Plugins.Administration.ContentFragmentManagement.ContentFragmentManagementUiService.ExportFragments(IList`1 options)
 --- End of inner exception stack trace ---