There was already a problem in the parallel configuration of the 2 Multiplus Compacts. One of the Multiplusses made a terrible noise. CMP242020000 HQ1341JBATN
After disabling the parallel configuration, I did a firmware upgrade to FW2618494.
Configured with VEConfig3 and MK3USB with the same settings as the other (working) Multiplus.
When switched ON this Multiplus still makes a terrible noise with only the battery bank (12V) connected. No loads, no grid.