Firefox plugin not working with local LT

I am running LT on localhost, which works fine with all kinds of plugins (including LibreOffice).
http://localhost:8081/v2/check?language=en-US&text=my+text works fine as well.

The plugin itself reports an error (“Cannot connect to http://localhost:8081/v2/check—please check your internet connection or try again in a minute (#1, code=0)”, and the log file looks like this:

2024-03-08 12:21:10.601 +0000 INFO org.languagetool.server.TextChecker Check done: 197 chars, en-GB (mother tongue: de)[auto], requestId: null, #26, null, 1 matches, 60ms, agent:webextension-firefox-ng, v:8.3.0, sent, q:0, h:2, dH:1, m:tlo
2024-03-08 12:21:10.645 +0000 INFO org.languagetool.server.TextChecker Check done: 200 chars, en-GB (mother tongue: de)[auto], requestId: null, #26, null, 1 matches, 104ms, agent:webextension-firefox-ng, v:8.3.0, sent, q:0, h:1, dH:1, m:!tlo

What am I doing wrong?

Have you started LT with the --allow-origin option?

Yes, I have --allow-origin-all in my command line.
As you can see from the log, I don’t have fastText, but that should not cause an error, should it?

--allow-origin or --allow-origin-all? It must be --allow-origin

I had --allow-origin-all, took that from here (my bad):

Great idea. I was thinking out adding another option such as --allow-origin-all but that’s simpler.
I saw that stackoverflow thread, and a bunch of other discussions about the bug.

However, with --allow-origin, LT won’t start up at all, and with --allow-origin '*' the results are identical to --allow-origin-all. I am using LT 5.3.

Please use a recent version of LT, we cannot help with old versions.

I thought I was pretty up to date… Do you have a version history somewhere?

The change log is at