Bad news I've got exactly the same problem with my H9 Max and it is now out of warranty…
When I switch it on it flashes with all its light and doesn't power up. But as we are several to signal this problem it seems there is really a flaw on this aspect on the H9 serie.
What can I do ? My H9 is really the center of my pedalboard and I really need it.
Well, there were a few different problems mentioned on this thread, and they're not all related. What you're describing doesn't necessarily sound like it's related to the power supply, b/c your H9 is flashing with all its lights.
Could you email firstname.lastname@example.org about this problem? I sometimes can recognize what's causing a problem based on the description of the H9's behavior, but I'm not sure about why your H9 would come on with its lights flashing. But in any case email@example.com could also give you the contact information for our French distributor if you need to send it to someone.