Ctek D250S DC-DC does this sound right?

Hi.

I installed the Ctek on my van. I have a solar panel mounted on the roof and another portable panel.
I installed it in such a way that I can easily clip the portable on with Anderson plugs.

This is how the CTek behaves, along with readings from the multimeter. (multimeter on the CTek at the input + - of the solar.
1) the "solar" light comes on, recognising the input from the solar panels, constant 18.8V
2) the charge light comes on after a few seconds, indicating that charge is now going to the battery. At this stage the power fluctuates - the multimeter changes all the time, reading down to 16V and even 14V.
3) after about ten seconds, the power lamp, solar lamp and charge lamp all goes out, the error lamp flashes once and the process starts again.

I have switched solar panels to see if that was the problem, and have double checked all connections in and out of the CTEk and all upstream and downstream.

Is the Ctek at fault, or is this normal? I have described it as best I can but further questions to diagnose the situation are welcome

Thank you in advance for4 the help
CJ