The debugging modifications I described recently helped to identify a problem with our Spartacus Remote Repository Tool
The problem occured only in a bunch of plugins. Our plugins repository sometimes contains directories with both a sidebar plugin AND an event plugin in that same folder. Even though the Spartacus Plugin properly fetches the files, it would only transfer the name of the directory depending on whether you first installed the event plugin or the sidebar plugin.
An example for this is the My Calendar plugin. In our repository, the files are in a folder called "serendipity_event_mycalendar". If you now installed that plugin via the Sidebar Plugin Manager, Spartacus would create a folder "serendipity_plugin_mycalendar" and of course fail when later trying to load the plugin from that directory.
Thus, an updated version 1.2 of Spartacus now fixes the problem, and returns the right target directory.
Note that you could also bypass the wrong behaviour of Spartacus in earlier versions by just installing the "other" plugin. That means if you tried to install the Calendar as Sidebar plugin, just try it again via the Event plugin.
If you care to notice why we often have plugins as both Event and Sidebar plugins: Basically we enforced a strict separation of those two plugins to make it easier for the user to differ if a plugin is shown in a sidebar, or if it enhances the flexibilities of Serendipity. And because only Sidebar plugins can be "moved around" graphically (via left/right side and placement) it would be hard to make an Event plugin have a Sidebar component and move that around.