I have a valve, actually a few valves in 3 separate part files and studies, that are symmetric by design.  They were setup as assemblies in which i brought only the parts i needed into a multibody part to clean up for simulation.  here is the interesting thing.  On one of the valves I setup the entire study including the symmetry fixture and it works and runs fine.  If i go back into the symmetry fixture to edit the definition(really just looking at it), don't touch anything, and try to just OK out of it i get this error.  If i X out of it the study runs fine.  With almost an identical part setup, just a smaller size, I get this error and I am not able to apply the symmetry fixture to it. 

I have tried to setup the model in different ways to see if it was my process.  The way everything was originally done was to use the FRONT plane to split all the solid bodies(there are 2 left after a delete body feature).  Thinking this was somehow the problem(although im not sure how all the faces wouldnt be coplanar with a FRONT plane split) I went back and made a sketch on the FRONT plane and extrude-cut away what wasn't needed.  Same problem.  Everything is restrained and bonded properly. 

So I'm a little baffled why the first study i setup and ran everything worked fine, but now if i try to setup the smaller valves in the same exact way, or even edit the definition of the original one I get the above error.  I am running SW2013 Premium X64 SP3.0 and just did a clean uninstall and reinstall.  Nothing changed between the setup of the original study and the new study with my system or any of the parts/assemblies.  The files are proprietary so i can't share them, but hopefully I can answer any questions. 

Thanks!