Blank page when switching to the custom theme...

Hi there, I'm developing a web site using the 1.5.1.3.1 version.
I did it on my MAC I using MAMP and everything was working fine.
My supervisor, still using MAMP on a MAC, just copied the files and imported my db dump... and everything worked fine.

Now he's trying to move everything on the real webserver (a MAC) and we get this issue, the website (front) loads just if we set the default theme, as soon as we switch to to the customised theme all we get is a black page, no errors, no nothing...

It has to be something related to the webserver configuration (it works on our local installations under MAMP) or a permission settings issue (MAMP doesn't really care about permissions)...

Any clue?
Thanks

Thank you so much... Obvious things never come to mind when you are stressed!

BTW, it was that php short-tags were not enabled on the php.ini!