You need to use your multimeter while the controller is flashing (that is important! not when it is not flashing), and tell us exactly what the voltage is
- right at the batteries,
- right at the converter's output terminals, and
- right at the controller's battery wire terminals.
Not using some display voltages--the independent hand held multimeter's voltage readings.
Also whether you are plugged in so you have converter (or inverter/charger?) DC voltage in the mix.
It is normal to have solar as well as converter DC to the house batteries at the same time while on shore power or generator.