All changes listed here, since June:

https://github.com/metabrainz/picard/commits/2.9.x/picard/webservice/__init__.py

Essentially break all plugins using WSRequest, since the previous versions did make use of Host, Port, etc. arguments. There is no fallback compatibility added on the code. There has been no notice about this on the changelogs of Picard, neither the plugins webpage state an update is required of some plugins for this. There is just V1/V2 api, but this is a breaking change on a non major release.