'The plugin does not have a valid header' on all plugins.

Hi,

Whenever I attempt to install a new plugin, whether it is via WP admin or by uploading via ftp, I experience the same problem(s) with ANY plugins I attempt to use:

1. First attempt of new plugin via WP: It downloads, unpacks and installs (according to WP Admin) . I then click on ‘activate’ and get “The plugin does not have a valid header”.

2. Second attempt of plugin via WP: Same as above but after the unpacking step it fails with ‘Destination folder already exists’.

3. Manual install: If I upload the folder to the correct location it just does not show up in WP.

The fact that this happens with all plugins would suggest to me that:

Has anyone out there got any advice here please?

Kind regards,
Arjun