Calibre indicated there was an update for this plugin. I installed it, and now I do not get the error message anymore. And the language tool gives me feedback on the text of the book. Thank you very much for that fast update.
There is one thing I do not understand. I have LanguageTool version 6.4 installed (downloaded it from
https://languagetool.org/download/ using the file LanguageTool-6.4.zip). When the LanguageTool plugin is finished, it displays on the screen the message: New LangugeTool version available: v6.3-branch
Do I maybe need to install the file LanguageTool-Stable.zip instead? I did not change anything on my computer since my original post.