I am using WordPress 3.8 (but I had the same error in 3.7.1) and when I click on Widgets from the control panel, I get a "500 - Internal server error." error from the Windows server. I do not know what is different today from last week - the website was working for a few years without any problems.
I put on DEBUG mode by modifying the wp-config.php file, but this particular page still gives me a 500 error.
I "reinstalled" WP 3.71 without any change, and installed 3.8 without any change.
What can I do?