I've got the same problem but instead of "500 Internal Server Error" I receive 404 Not Found
I'm not surprised test received 404 error because in fact, no 000000 directory exists in my l/wp-content/cache/minify/ directory, hence there is no w3tc_rewrite_test file.
All settings are as recommended. Cache directory and subdirectories chmod is 755
I wonder why? What is wrong?