Trying to squash out the obvious:
* Did you change anything else ? Is WP exactly the same as before the theme switching ? If so, try disable or temporaily undo’ing that.
* I’ve got a feeling you or a component/extension has screwed up your .htaccess. Try renaming it temporaily to .htwhatever and see if you still get the 500 internal server error’s. If so, then it’s up to the next person to post in this topic :D