I have the same problem on my OpenSUSE 42.1; my Sigil 0.9.5 is compiled using
this repo's srpm (0.9.4, with source code change to 0.9.5), but I remember that I installed the binary rpms before 0.9.5 from it and got the same result.
I use FlightCrew 0.9.1 and ePub3-itizer 0.3.6. I've used the test plugin too.
1. if I run the plugin from menu->plugins, the "A plugin with that name not exist" message appears.
2. if I set the plugin to the shortcut button then run the plugin from the button, the plugin works.
3. the problem only appears when I'm in KDE5. If I'm in IceWM (preinstalled by OpenSUSE), there's no problems.