I probably spoiled my installation.
I uploaded a plugin that was incomplete, and activated it. When I tried to open wp-admin/plugins.php or wp-admin/plugin-install.php, it prompted me to download the files instead. So I deleted the plugin -- and wordpress told be that it deactivated the plugin as it doesnt exist.
Now each time I upload a plugin, it does not open plugins.php but prompts me to download it. Also, plugin-install.php never opens (always prompts for download)