Question about Marshall Major III Bluetooth

Hello Grxnsx,
I can't say anything else than Dominique.arjanti.boys: Switch off (the control light goes out after it has lit up red), then switch on again by keeping the bronze-coloured button pressed SOLANGE until the light changes from a white light to a blue light. This is then, as you correctly write, the pairing mode. For me, this works without any problems with different devices as music sources (iPhones).

Have you tried the following?
Press and hold to switch on, but hold when the headphones switch on, then they switch to connection mode.