The reported bug is solved. Please test it with the tomorrow nightly.
The start of the dialog is sometimes slow, especially, when a huge document is just opened. LT generates a cache in the background. The check dialog should speed up with the time a document is opened.
LibreOffice 7.4 now supports connecting to a LanguageTool server natively. I suggest you run a local server and connect to it, without any need for an extension.
Thanks for your response. However, I am just a user of LibreOffice Writer who wants grammar to be checked. The easiest is the extension. I am not familiar with setting up an own local server just to check grammar. On the first sight, it sounds more complicated than clicking on an .oxt file…