Your signature seems out of date ;-)
Make sure that your text is not flowed text. Select it, do Text > Convert to text for any flowed texts you have.
If that isn't sufficient, just turn them into paths. Select text, do Path > Object to path. This will work with Gimp.
Can't tell you why the Gimp conversion fails - Gimp is installed on the same computer, right? And the command line is working for gimp?