Plasmate isn't aware of runtime deps, those are pykde4 and ruby bindings. Plasmate will be configured, compiled and installed without any complains but it will fail in the runtime. Especially for the plasmoids which have a previewer Reproducible: Always
Took me a long time to figure it out. It should be easy to add it to the dependencies. If someone could point me in the right direction I would even do it myself and create a pull-request.
That's a downstream bug, please report to your distribution to correct the package to pull in the needed dependencies on installation.