Well,
I have exact same problem as far as output of error:
YOURS:
[root@Winux bin]# ./fluxbox
Failed to read: session.tabs
Setting default value
Failed to read: session.slitlistFile
Setting default value
Failed to read: session.groupFile
Setting default value
fluxbox.cc: Loading groups ()
Failed to load groupfile:
BScreen::BScreen: an error occured while querying the X server.
another window manager is already running on display :0.0.
Blackbox::Blackbox: no managable screens found, aborting
Fluxbox: Unknown error.
EXCEPT yours has one line different:
root@linux1 root]# fluxbox
Failed to read: session.tabs
Setting default value
Failed to read: session.slitlistFile
Setting default value
Failed to read: session.groupFile
Setting default value
Failed to load groupfile:
BScreen::BScreen: an error occured while querying the X server.
another window manager is already running on display :0.0.
Blackbox::Blackbox: no managable screens found, aborting
Fluxbox: Unknown error.
diff = fluxbox.cc: Loading groups ()
So you might be one step closer, but I can give you this info....
Fluxbox is not a "session" to be managed by Gnome. You should login to the graphical session of Blackbox, THEN you should be, should be... able to select by Right Click -> Others => Start Fluxbox
It is a Window Manager, but not an entire Desktop environment like Gnome is. The Blackbox is the Desktop environment that you load and you can pick and chose different Window Managers once the session has started. Because it uses the same basic framework upon which the 0:0 display is being served.
Try going into Blackbox, then right click, and chose Other -> you should see Start Fluxbox
Unfortunately, event though I installed via TarBall, and I got no errors, it still won't appear as a selection for me, and attempting to run it by command line... from within Blackbox, I get the same error as you got above.
Also, you might try just firing /usr/bin/fluxbox from init 3 and see if it works... it may start X in Flux that way... but it hasn't worked for me yet either... at least you are seeing that one difference in errors.
GL