I allready reply to myself, but any comment would still be much appreciated.

So.
I re-lowered the quality boost sliders to default zero, and checked the re-use light box, and the second frame started rendering after 18'15".

From what I can see, the GR solution seems to be stable ! =D

 

Have to leave soon from my place, I'll leave the render go and see after work how it went tonight.

 

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