Unable to access theme options

I had an initial install of 2.86 and upgraded to 2.9, however this issue occurred in both versions.

When installing and activating any theme that has theme options, I unable to access this via:

Appearance -> ‘Theme name’ options

The URL it tries to go to is:

http://www.mydomain.com/wp-admin/themes.php?page=functions.php

And brings up a message saying:

“Cannot load functions.php.”

On other themes it goes to this URL:

http://www.mydomain.com/wp-admin/themes.php?page=dashboard.php

And has the error message:

“Cannot load dashboard.php.”

I entered a support ticket with my hosting company as I thought it may be a permissions issue, something I am not familiar with at all. I was also getting errors when trying to access the editor link via the appearance tab, however that was solved as per this thread:
http://wordpress.org/support/topic/344250. They could not see any issues with my permissions or content so I am at a loss.

This occurs on every theme I have tried to install and is extremely frustrating as everything else works apart from not being able to access theme options.

Does anyone have any idea what is up? Any help is greatly appreciated.

Thanks, Matt