Hmmm I am running out of ideas!
Is the Converter definately the BL1611 version? – the BL1614 will not work.
Have you tried the BL3500 in L.OPT 6 – that’s the default and works in most setups. I am not sure if you can put the BM7000 in A.OPT 0 while you try this, unless you can temporarily move the BL3500 to another room.
If you have tested the Aux and ML cable elsewhere, you could do a test of the ML/BL3500 part of the system by setting it up as explored in this thread (page 4 onwards): https://forum.beoworld.org/forums/topic/beolab-3500-and-1611-converter-settings/page/4/
If you have the right cables, you could test in accordance with matador’s excellent diagram:
