If you have Steam and Fallout 3 in a non-UAC directory, especially if you have configured the directories for Steam, MO, and the game files for full control by all users, there is no need to use "run as an administrator" with MO. In some cases, as Tanin pointed out a while ago, this could actually cause a problem. (I'm not sure if this is an issue for you).
If the mods are setup for use with MO then they are not in the actual game Data folder, so they will no be visible to Steam or any executable in the Fallout 3 Goty directory unless the game is launched by MO. Having MO in the background doesn't help, it needs to run the game launcher that is used.
Try running an MO profile with only the vanilla files (no additional mods) and see if FO3 can start using the FO3 Launcher or FOSE. In each case look at the logs produced in the Mod Organizer\logs folder. The log is small so you might want to post it here.
I have tried running vanilla FO3 via MO
but have the same issue with the same error.
Using the FO3 launcher causes the same error (5:0000065434) but using fose_loader loads the game fine (though obviously without mods).
The issue of running MO as administrator should not be under-estimated. If Steam is started as an administrator's process and MO is started as a regular user process, MO won't be able to 'see' the elevated level processes of the system, which in this case will include Steam.
This is why it is stated over and over again to install Steam and any games that use it, along with MO, outside of ANY UAC folders, thus avoiding the need to use administrator rights to start one or more processes.
One last thing, the original issue was resolved when the user applied the latest update from Steam, have you likewise kept Steam up-to-date?
I have tried running Steam as Administrator and MO
as Administrator (steam is not installed in a UAC
directory) but crashes with the error: P:0000065432.
I have the latest version of Steam updated today.
Latest log (ModOrganizer_15_05_16_13_40_2.log):