If I click on the help button in the Plugin Manager then it shows me the help for that plugin. This usually works fine.
But if I enable and disable a plugin a few times and click on help after that, then geany will crash quite likely. This happens no matter if the plugin is using the old or new plugin API. Also the kind of help shown does not seem to make a difference, e.g.: - the Latex plugin opens a info dialog - other plugins open a webpage
In both cases I could re-produce the crash. I tested with a quite actual development version of geany under Ubuntu 16.04, GTK3 build.