I received them yesterday : a lot of original Kodak series 6 filters in good condition. But the red filter suffers from heavy separation. It's the third red filter I find like this. Bad luck or do red filters usually age bad and why ?

Bad luck and poor storage/cleaning.
These filters have a layer of gel sandwiched between two pieces of glass. Moisture will destroy them. Be careful cleaning them. Always put the solution on the cloth/tissue. Never apply it directly to the glass. If it gets between the mount and the glass, well.....
For what it's worth, I've had bad luck with the yellow ones.

I agree that it probably isn't an issue of the filter color, but of infiltration between the sandwiched elements. I've seen this on several different series VI filters. It took me quite a few searches to finally come up with a perfectly clear polarizing filter, but hey, with this stuff the search is all part of the challenge! When you finally do find it, isn't it much more satisfying than simply walking into a photo store and buying it off the shelf? Or so I keep asking myself.

Well who has a usable red filter for me, please ? Just kidding ! Thanks guys.

I'm really tempted to glue a 43-49 step-up ring onto a Series VI retaining ring and stop the Series VI filter battle. The box of filtes I got was so musty & mildewed I automatically washed each filter in soap & water...now I read what a bad idea that is...

Are you sure about the 43(?)-49 step ring? The series VI thread is 44mm. Have you actually had success using a 43 into a series VI slip-on? If so, those must be sloppy 44 steppers I'm using; all three of them! (44-49, 44-52, 44-55). On second thought, maybe that's why you said "glue."

Don't feel bad about washing the filters in soap and water. That's what Rigby Reardon (Steve Martin) used on his pistol in "Dead Men Don't Wear Plaid"---seemed to work fine for him!