Good afternoon Christophe.
Sorry about the delay in my response, have been away for the weekend.
Can’t think of anything else to suggest to you re your volume adjustment problem, I guess you have tried turning off the power changing the dip switch positions then power back on?
Also I have read in a post in General Discussion started by Gilles about volume control problems, and that the crystal on the V1.5 is very sensitive and if touched by accident that this can cause issues with sound. I am unsure how this is resolved as the post is in French and my French is very poor. It may be worth you having a read through it.