I recently moved the demo repository of phpQGISrepository (https://plugins.stuyts.xyz/) to a new webhost and since then it didn’t work anymore in QGIS 2.18. In QGIS 3.x and in all webbrowser it still worked just fine, but in QGIS 2.18 it gave a 400 error (Bad Request).
It took me a while it figure out what the problem was, but apparently the problem was that the webhost gives that Bad Request error if the url doesn’t contain a trailing slash. So the solution is very simple: in QGIS 2.18 you have to use https://plugins.stuyts.xyz/ instead if https://plugins.stuyts.xyz and then it works just fine.
phpQGISrepository can be downloaded from: https://gitlab.com/GIS-projects/phpQGISrepository