Cannot install any new plugins

HI.

I have inherited a site which was created by someone else (who i cannot contact).
it has multiple plugins installed, and all are working fine.
BUT – if i try and install a new plugin – I get the message:

“The plugin does not have a valid header.”

I have tried various plugins – and the same error with each.
Also, if I try installing the same plugins on another WordPress installation, it works fine.

I’ve tried installing via FTP as well as through the WordPress installer.
Also, the plugin is not in a suddirectory – which is a common solution when googling the problem.

Help!