Quote:
...I've put them in a py-plugin-folder in my personal folder of gimp 2.8...
That bit is a little concerning. Why do you have a
py-plugin folder?
Questions.
Did you create this folder yourself? If so did you add the location of the
py-plugin path?
Do you have a regular plugins folder in your Gimp profile
~/.gimp-2.8/plug-insGo to Edit -> Preferences -> Folders -> Plugins to check what locations are enabled.

This one I have an additional folder
plugins-test that I added myself. If you do not have
py-plugins showing then put any plugin
binary or
python in your Gimp profile
~/.gimp-2.8/plug-insMaking it executable. This is kubuntu 16.04 but regular ubuntu will be similar.
Use your filemanager
Toggle hidden files alt+. (alt-dot) or use the view menu, show hidden files.
Find the plugin
Right click
Go to properties
Make sure the executable box is ticked.
example:
