If you can not login you have not properly installed the application to integrate with xenforo. That would be my response. I would suggest you recheck the tutorial I posted. Make sure the usergroups table was indeed cleared and you have reinstalled xenforo correctly. There should not be any mysql errors or displayed errors if everything is installed correctly. If you login and it looks like the username and password have been accepted but your not logged in then I would suggest you check the cookie settings in the database. In your browsers option menu under security depending on the browser you can view your cookies and determine things there what might wrong.
We do offer installation service if you wish our company to install the gallery properly.
Like I said without futher info all I can do is guess. You can place a url and a test user login if you want further comment but as long as you do as posted above you should be fine.
Usual culprits to not being logged in are
1. authentication works but not logged in. cookies settings are wrong.
2. you have reinstalled but there is a message
"We're sorry, but the system experienced an unrecoverable error. Please try again later"
That usually indicates wrongfully inputting forum info maybe wrong table prefix or database info. It indicates that we can not grab the user info from the database.