@Sebastian-Roth You’re welcome …
For info, my switches are HP Procurve and I used these links to configure them !
https://community.spiceworks.com/how_to/26481-enable-igmp-on-hp-procurve
http://whp-hou9.cold.extweb.hp.com/pub/networking/software/AdvTraff-Oct2005-59908853-Chap04-IGMP.pdf
Now, Multicast is working properly : I imaged 3 clients at 1.4 Gb/min, next I will try 10 or more !