Francois, I really have no idea just what you are doing, or why.
You "appear" to have a mixture of Progressive and interlace controls in conflict with each other; a VBR that is set to 15 MAX and your Average is 15 too as well? I've not come across that; What are your Media properties?; You have 32bit float I'm guessing for better transition options - yes?
This is far too complex for me to understand. The "breakup" of that picture I suggest are the least of your issues here, and could/most likely be the result of your underlying Project<>Render settings depending on your media . . . . I'm truly very very lost. Sorry....
Video Content Creator and Potter
PC 7 64-bit 16gb * Intel® Core™i7-2600k Quad Core 3.40GHz * 2GB NVIDIA GEFORCE GTX 560 Ti
Cameras: Canon XF300 + PowerShot SX50HS Bridge
Select a region where the problem occurs and then try an AVC render template to see if the problem remains. If you get the same lines during the fade, try inserting a solid black Media event on that track at the point the fade begins, so that it fades to the generated media rather than the blank timeline.
My eldest had tampered with most of my presets. Was so used to using them that I was not even paying attention to what was in the box. Thx to Graham's reply regarding the bit rates, got me to stop and look closer.
Note to self: Never assume anything...and put a bloody password on the work station !