We have just released LanguageTool 2.9.1 for LibreOffice/OpenOffice. It fixes the "osl::Thread::Create failed" error that some users have experienced, especially on Windows. You can get it from https://languagetool.org or, if you have LanguageTool installed already, directly in the LO/OO Extension manager dialog (right click on LanguageTool, then "Update...")
As this update affects only the LO/OO add-on, the latest version for the stand-alone version of LT continues to be 2.9.
SHA-256 checksum of LanguageTool-2.9.1.oxt: e124cd6cefc58ee3a221e83daeb6244b6019dc97f6aaca40d6d369140162193a