Plugins, Updates… Not working

Hi all!

I recently migrated my standalone-website to a multisite, which works as it approximately should.

So I had https://www.hetwittepaard.be (let’s call it site 1 from now on) as a standalone and I’ve added https://jeugd.hetwittepaard.be (site 2) as a kind of ‘child’-website.

However, when I go to My Sites, I am able to ‘Network Activate’ my plugins. I now can see all the plugins being active on site 1. However, on site 2 none of the plugins are neither active nor available. Even worse, it says: The plugin XXX has been deactivated due to an error: Plugin file does not exist.

Also I am unable to update site 2 to WordPress 4.0, whilest site 1 was updated without a problem…

Themes don’t work either.

Please help!

Thanks in advance!