I have the exact same problem. Any plugins I can find, has the same result when trying to install them: "...not supported...".
This is after I have formatted my desktop disks, and started to reinstall all apps. Before formatting, I was using some Qbit version starting with 4.1.X, without problems. Then I also formatted another desktop, and found no problem when re-installing the latest Qbittorrent version.
The only difference was that the one that works, is a W7Ultimate version of windows o/s, and the the one that does not work, is a windows W10Pro version. I then uninstalled Qbittorrent and re-installed a version 3.x.x of it. That also did not get the plug-ins to install. Same message of " ... not supported...". I then upgraded it to the latest version of qbittorrent, but still the plug-ins would not work.
Can anyone help?