This is from a completely fresh install.
When trying to Install extensions, it downloads just fine then displays a message saying Error encountered. Details
and the Details link shows this message:
An instance of task failed successfully
perhaps?!
When you then refresh the page, you are able to simply press on Activate
and the plugin is activated correctly.
This occurs with third party extensions (e.g. ACF) as well as Themeco Extensions (e.g. Custom 404)