If anyone else is interested I had same problem with 14/Nadia also, but with a twist. In addition to user/pass issues, also had some 'login loop' problems on a 2nd install attempt with v14. Tried both Cinnamon and Mate, same results on both. On last attempt at install, thinking I had fix in hand for a possible nvidia issue (nomodeset), the computer name / user name / pw (not certain what screen is officially called) setup got real buggy. Computer name field was generating 'name already in use on network' (or similar), even when I tried name options nowhere close to anything that is or has been used prior. After several attempts I decided to go back to 13 Mate and it's working fine. Really wanted to give Cinnamon a try this time, but it can wait til later...
Not asking for a fix, just want to share in case it might help with a patch/fix down the road.
System info: New build, dual boot Mint/W7
Gigabyte GA-Z77X-UP5 TH
Intel Core I7-2600K Sandy
CORSAIR DDR3 1600 32GB (4 x 8GB)
MSI N660 Ti PE 2GD5/OC (NVIDIA GeForce GTX 660 Ti)
Intel 120G SSD - 320 Series
WD Black 2TB