It can take up to 48 hours for the linking between AdSense and YouTube to be completed on the YouTube end of things, so that is a possible reason. Although that doesn't usually generate a 500 error, as far as I know.
Are ads still appearing on your videos?
Also, when you say you switched your AdSense account, can you explain what you did exactly? Did you simply switch from one fully active AdSense account to a different fully active AdSense account? or did it involve cancelling the AdSense account that was originally associated with your YouTube channel?