OS X Server: Wiki embed youtube video on page
I had the same problem.

On the Server load terminal.app in your Utilities folder, and type in:

On Mountain Lion:

sudo defaults write /Library/Server/Wiki/Config/collabd.plist FiltersEnabled false


On Lion:

sudo defaults write /etc/collabd/collabd.plist FiltersEnabled false


You may need to turn the Wiki Service off and on afterward for the settings to take affect. You can do this by typing:

sudo serveradmin stop wiki


then

sudo serveradmin start wiki


in the Terminal.