My board isn't working properly...

I am trying to login on this board, but every time I try, the screen flashes and sends me back to the login page. Please help.

Not sure, but in some L4T releases, trying to login in GUI with a user not belonging to group video has led to similar symptoms.
Is it the same with user nvidia ?
Can you log in from console ?
Any errors logged in messages from kernel or syslog ?

An indirect way to lack permission for group video is if sudo does not work…does “sudo ls” work? If not, then permissions were not preserved during creation of the loopback image during flash (which of course means you need a serial console or ssh login or text console for this to be tested).