Plugins are downloaded but not available

I had some problem when migrating from version 10 to 11.

I have some plugins that are note in the installed plugins list, but that are displayed in green and with a 'downloaded' status in the plugin repo browser.
I can't neither install/uninstall these plugins and worst, I can't use them.

Anyone have an idea (more sublte than trashing away all my plugins) to get these plugins at work again ?

Thanks,
Guillaume.