There is a problem with the 'Become Observer' button in online games, and I thought perhaps it could be reproduced if you were to, say, 'Become Observer', save, quit, and reload the game. It was a long shot, and I can't personally reproduce this. Unfortunately I don't believe there is any way to unlock the other side - however, you _could_ try this:
1) Change your password while your game is open
2) Save the game
3) File -> Close Game
4) Change your password back to the original one
5) Load the saved game
6) You _may_ now be able to join as 'Allied' - but not 'Axis'. Join 'Allied'.
7) Retire, and click 'Join another side', '<observer>'
Save the game
9) Close the game
10) Change password to your new one
11) Load the save game
12) Both sides available? Maybe?