when you have access to phpmyadmin, you should do this:
- open phpmyadmin with your joomla databse
- open the _extension table
- edit the item for the system plugin (see pics)
afterwards should you have access again to the backend.
Seems that you had let installed the old 2.5 system plugin from jDownloads.
Okay when you need not more jDownloads, you should uninstall ALL jDownloads extensions. Please search in the extensions/manage page to 'downloads'. Then should you get listed all.
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