Thank you for your post.
I have installed FileMaker Server 13.0v1 on a Mac OS X 10.9.2 server using admin/admin, and I have no issue. I am able to login, plus I'm able to upload databases to that server using admin/admin.
Perhaps you could provide me the exact installation steps so I can replicate the issue here.
I had a clean install of FileMaker Server 13, no previous installation of FMS on the machine. I first ran the setup using fmg/fmg as credentials and ran into the issue. When I noticed that a also could not upload databases due to a password error although loggin into the console worked fine, I decided to start from scratch. I deleted the four config files, restarted the adminserver and re-ran the setup, this time using admin/admin but getting the same errors. Running the setup a third time I used my first name as username an some random password. Now the username appeared in the authentication section and I could change both password and username. Also the database upload now worked for the first time. I just wanted to re-run a setup and cover my steps with ScreenFlow but now even admin/admin worked the first time. Maybe someone could test his with a fresh OS installation. The machine is a Mac mini running OS X 10.8.5 and Server 2.2.1.