Thank you a lot for your reply!
SDLXLIFF to Legacy Converter has done the conversion to ttx without problems!
Before of that, I've used the utility TTX, but it has returned an error message because I hadn't selected any filter for the conversion. But actually it is not clear to me how to create a filter for the conversion, therefore I've tried to use SDLXLIFF to Legacy Converter.
Bernard Lieber wrote:
You've got 2 options with 2011, either you TTX your Word file available under the OpenExchange folder (SDL Start) and create a project and when done export as Trados TagEditor
Download SDLXLIFF to Legacy Converter and convert your SDLXLIFF file to ttx.