I found that this issue also occurred in openHAB 4.2.3.
I am not familiar with Jetty client library and its idiosyncrasies, but I suspect it doesn’t handle the Upgrade: h2 header that the server is sending back. It returns status 200 without following up on the upgrade header.
I’m not sure if this is something that can/need to be fixed within openHAB (maybe it is, but I’m not knowledgeable enough about it / Jetty, and I haven’t dug deeper).
The current Jetty client in use in openHAB is old, and an upgrade to a newer version is not imminent. See discussion here: Upgrade to Jetty 10.x (or Jetty 11.x) · Issue #3315 · openhab/openhab-core · GitHub
You can continue using your current approach of calling wget. The other solution I can offer is using JRuby, because it has its own HTTP client implementation.
In JRuby, your code would look like this:
json = Net::HTTP.get(URI("https://www.myjudo.eu/interface/?group=register&command=login&name=login&user=NAME&password=MD5hash&role=customer"))
logger.info json
If you need help in converting your entire script, I’ll be happy to offer assistance.