I think almost everybody responding to this topic has been looking at the wrong cause. The issue was not a wiring problem, but a configuration problem.
You can NOT use the multiplus to monitor your power flow if there is ANY other input to the system. The multiplus only knows about power that flows into and out of itself. A solar controller in the system can never be measured and reported on by the multiplus. This is EXACTLY what you are seeing. The voltages show that the batteries are being charged yet the current flow is zero.
You need to go into the multiplus set up and be sure that battery monitoring is turned OFF, and in the CERBO system be sure that all the monitoring is done by the BMV. This is a requirement for the system that you have. If you don’t do this it will never be able to work. Read the manual for the cerbo and the multiplus carefully. It is all explained in there.