Dear Rémy,
A particular change in BaseX 11.4 that took me a while to find, is a change in http:send-request. So this is only relevant if you use that function. If you send a JSON body, previously you had to serialize it, like:
http:send-request( <http:request method='put'> <http:body media-type='application/json'/> </http:request>, $url, serialize($body-map, map{'method':'json'}) )
This has changed, and the right way to call this is now:
http:send-request( <http:request method='put'> <http:body media-type='application/json'/> </http:request>, $url, $body-map )
If you use http:send-request, I hope this will save you a bit of time.
Best regards, Nico Verwer