Cant install plugins and themes

Hi Guys,

Just wondering how I can get around a problem.

I installed Wordpress MU a few times already .. never any problems.. without buddypress it works fine using subdomains.

Now, when I try to install themes or plugins using the wordpress uploader or via FTP ( using correct paths as indicated ) they never install.

Eg .. with themes, I get:
eg: Theme name: andreas Stylesheet is missing.

With plugins, It just wont install .. but any other wordpress plugin works as expected.

How can we troubleshoot this problem?

Any assistance would be great.