I hate to be so simple, but double-check your caps lock key. I have on occasion, accidentally had the caps lock on when I created the root password. Toggle back and forth to see if that is the case.
If that fails to work, then go into runlevel 1 to change the root password.
How To Go Into Runlevel 1
- At the grub screen, hit enter.
- Select the proper kernel (if you have more than one listed) and hit the "e" key.
- Again, arrow down to the line that starts with “kernel” and hit the "e" key again.
- At the end of the line that reads “rhgb quiet,” place a 1 there and hit enter.
- Example: . . . rhgb quiet 1
- Now back to the previous screen, hit the "b" key to boot.
Once in runlevel 1, change the root password to any of your liking.