There are multiple options for the username/password defaults depending on when it shipped and what factory firmware is on the router when you defaulted it.
Possible options could be:
username/unique password on the label
admin/unique password on the label
If you are having issues, use SSH rather than the web interface as caching issues can cause problems as well. On newer software there are incorrect password timeouts that also simply show as a failed user/pass combo so be careful how many attempts you make.