I remember seeing issues with the infinity friendly URL and the latest DNN 6.1
I recommend you completely uninstall infinity (make a backup of your site first.) then test after "restart application" in host settings. I know you "disabled" it bu I bet the files are still referenced on the site load.
* AND clear all temporary internet files on your client. Try a completely different computer (that has never browsed your site) as a true test.
Take out other variables like this, to bring it to JUST DNN and Smith Cart.... it's up to you to test this. Backups are your friend.
It probably is the order of files the DNN has changed.... doubtful if that will change any time soon, DNN is leaning on "Make a backup and restore if DNN 6.1 does not work" and they will continue to lean on this and only focus on the Paying PRO customers. Us community guys just have to be more careful and wait for Pro users to experience the same so it get's fixed.
Good luck!
- Mike - www.itedgecrm.com