Pages are broken – programming error?

customize page and header pages give me the errors below. other themes work FINE. What is the problem here??
Most likely causes:
?The website is under maintenance.
?The website has a programming error.
What you can try:
Refresh the page.
Go back to the previous page.
More information More information
This error (HTTP 500 Internal Server Error) means that the website you are visiting had a server problem which prevented the webpage from displaying.
For more information about HTTP errors, see Help.