I'm on Lucid x64 and the way to reproduce this is easy:
1) delete ~/.eclipse
2) start eclipse as a regular user
3) try to install some third party plugin
4) restart when prompted and the plugin is not loaded, although appears as installed.
I checked the configuration with the legacy installer and the status for the custom plugins is "Not included in the runtime".
I'm on Lucid x64 and the way to reproduce this is easy:
1) delete ~/.eclipse
2) start eclipse as a regular user
3) try to install some third party plugin
4) restart when prompted and the plugin is not loaded, although appears as installed.
I checked the configuration with the legacy installer and the status for the custom plugins is "Not included in the runtime".