I subscribed to LanguageTool service using an email address which containted the character "+"¹. It seems that the integration of LanguageTool whithin LibreOffice does not support the usage of this character, as when I used another email without it containing the “+” character, LanguageTool was working perfectly.
Yes, I refer to this bit of integration within LibreOffice.
But it’s weird, I was redirected to this forum when I submitted a ticket to LanguageTool. ("So if there is a specific problem in the integration, please contact our community forum directly to reach the developer: https://forum.languagetool.org/ "). I guess there is a communication issue somewhere.