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I found two strange points.
1) It is described in the "USB Interface Association Descriptor Device Class Code and Use Model" that the Device Descriptor using IAD should have class codes as follows.
USB Interface Association Descriptor Device Class Code and Use Model
2) It is described in the "Interface association Descriptors ECN" document that the bFunctionClass in the Interface Association Descriptor (IAD) must be specified.
Interface Association Descriptors ENGINEERING CHANGE NOTICE (ECN)
The Device class code is described in the "Defined 1.0 Class Codes" WEB page. The class code of CDC is 0x02.
Thanks for your feedback. It did not solve the issue, but at least the descriptors are cleaner now
In the meanwhile, I was able to find the issue. It's the combination of Win7 and Intel USB 3 controller.
My understanding is that Win7 lacks from native USB 3.0 support, each HW manufacturing providing its own USB 3.0 stack.
On the same PC, I installed another USB controller (from asrock, ASMedia chipset) and everything was fine
On another PC running under win10, using intel USB controller, it is also fine.