I've uploaded a plugin through the Extension Registration panel, but I've used the wrong target version of SMath; therefore I've deleted the pluginfrom the "uploaded files" panel and I've tried to upload again it.
This is the error message that appears:
Uploaded extension was already registered earlier by another person.
Wrong twice:
1) The plugin was uploaded by me;
2) The plugin was registered but should be unregistered.
Anyway compiling another plugin all goes fine (except that I can't see the plugin from SMath or in the Registered Extensions page)