LT and firefox giving a error message I donot understand

I have been working with LT 3.5 for quite some time with Texstudio (TXS) 2.12.2. However the connection between TXS got cut somewhere. Now I am trying to reestablish the use for txs. But I encounter problems:

Starting the LT sever in its directory gives:

>java -cp languagetool-server.jar org.languagetool.s
erver.HTTPServer --port 8081
WARNING: running in HTTP mode, consider using org.languagetool.server.HTTPSServer for encrypted connections
Setting up thread pool with 10 threads
Starting LanguageTool 3.7 (build date: 2017-03-27 10:50) server on http://localhost:8081...
Server started

This seems ok.
Using firefox and the line from the wiki:

http://localhost:8081/v2/check?language=en&text=my+text

shows:

2017-04-30 11:12:45 Check done: 7 chars, en, #1, null, 1 matches, 2549ms, agent:-, sent
2017-04-30 11:12:45 An error has occurred, sending HTTP code 400. Access from 127.0.0.1, HTTP user agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:53.0) Gecko/20100101 Firefox/53.0, language: null, Stacktrace follows:
java.lang.IllegalArgumentException: You're using an old version of our API that's not supported anymore. Please see https://languagetool.org/http-api/migration.php
        at org.languagetool.server.LanguageToolHttpHandler.handle(LanguageToolHttpHandler.java:122)
        at com.sun.net.httpserver.Filter$Chain.doFilter(Unknown Source)
        at sun.net.httpserver.AuthFilter.doFilter(Unknown Source)
        at com.sun.net.httpserver.Filter$Chain.doFilter(Unknown Source)
        at sun.net.httpserver.ServerImpl$Exchange$LinkHandler.handle(Unknown Source)
        at com.sun.net.httpserver.Filter$Chain.doFilter(Unknown Source)
        at sun.net.httpserver.ServerImpl$Exchange.run(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)

which is strange.
Win7, firefox53.0 (64bis), Language Tool 3.7

That’s strange indeed. I cannot reproduce this with LT 3.7, so I suggest using that.

Did you notice I used LT 3.7 already?

I have texstudio back with LT to provide control over spelling and grammer. Sorry for the noise. The error, however was independently.