I have found that in LibreOfffice it is necessary to confirm that the Lightproof Grammar has been really disable, not only in the Module Pane, but in every language selected through the "Edi"t option of this Pane. Unchecking the LightProof and checking Languagetool the Languagetool.odt test-file now is working fine.
This problem has been solved that way.
Thanks