Happens to me all the time, I'm moving along setting up my Irradiance Particles and get a nice setup going and then I decide I want to see what it looks like with FG or GI or FG and GI.
Gotta turn IP off for that to work of course, so I do and try some FG, and all is nice and working.
Then I go to turn GI on and get the error message "Irradiance particles are used, switching globillum off" and while the FG still does its thing, the GI doesn't.
So I go back to check out to see if truly ALL the IP stuff is off and it is. Not that that should matter any, because FG shouldn't work anyway, if IP was actually still on.
Still though, I can't get GI to work, and get that error message.
However, if I save the scene at that point, close Softimage then reopen it and the scene, the GI works fine without errors. Even though nothing has changed in the Render Settings and all I've done is close and re-open.
Anyone else? Is this a bug that needs to be reported or is there something else I could do? It seems like some kind of memory error, like when switching to GI, Softimage somehow isn't being made aware that IP was in fact turned off, while it does in fact know that it's okay to do FG. *shrug*