IM/LC - Prepass - Render suddenly slow

I have a animation of 450 frames that i need to re-render 9 frames because one of the renderslaves had problem finding some textures. Not a problem i thought, as i already had made the Prepasses!
BUT ... Just putting in the 9 frames in Range and pushed "2- Render anim", got a rendertime of 22 hours of a frame that should take around 20 minutes.
I also copied the MAX-file from the Backburner Manager and used that max-file, same problem.

Have for the last three weeks used prepass and render, no problem. And as this is the last weekend before delivery on monday and don't get it.
Have now rendered the frames without prepass, but i don't get what is causing this.


Any suggestions?

I had a look in the file that was submitted to Backburner, could not see any keys on subdivs on the lights (i have 2).
I am using Vray 3.3, with Vray Goodies turned on.

The strange thing is that i extracted the file from Backburner Manager (the file that has been rendering for a few days, without any problems). If i submit the exact file (this file has applied Solid Rocks settings) i get super-long rendertimes. The problem seems to be that it is using pre-calculated IM-files, if i change GI to bruteforce/lc everything is ok.
It seems that using the same IM-precalc files that i have been using for the last days the problem occurs.

i re-renderd ~170 frames last night with both pre-pass and render, everything worked as expected.
So this morning i loaded the file submitted to Backburner and tried to recreate the problem i had.
The last frame took 20 minutes, so rendered as expected.

So i don't know what my initial problem is as it worked today. Could something changed in the scene (as i am using Xrefs).
Would the rendertime be very long if i make a prepass and then change something in the scene and try to render?