I was having this same problem and alanft was right. It was some invalid code in one of my widgets.
If you're unsure of which one, you could try my testing procedure.
- Remove all widgets that use Widget Logic.
- Enter your URL into http://web-sniffer.net/, select "POST", and Submit.
- Ensure you're receiving 200 OK instead of 500.
- Activate your widgets one at a time and repeat the above test until you activate the widget that produces the 500 error. (You may have to refresh your cache after each widget change.)
This is how I found which of my questionable widgets had the error. Then once I looked at the widget logic, I realized I had a misplaced character. Once I removed that, saved the widget, and refreshed my cache, the web-sniffer test returned 200 OK again.