Charge controller same volts "input" as "output"
Based on the additional information in the comments and the not very helpful information from the datasheet and user manual, I'd say your system is working correctly.

Your panels cannot provide the needed power to charge the battery at 14.4V. Solar panels have a very pronounced dependency between output voltage and output current.


(taken from Wikimedia commons, by Squirmymcphee)

This shows, for a single solar cell, the dependency between output current and output voltage. \$V_{OC}\$ is the open circuit voltage which you measured to be about 19 to 20V. \$I_{SC}\$ is the short circuit current, which you haven't actually measured, but you are quite close to it I guess.

A battery has a very low resistance (often in the order of some milliohm) and as such it will draw massive amounts of charging currents. The voltage you can measure at the battery terminals is the chemical voltage plus the voltage generated over its internal resistance by the charging current.

So your solar panel will provide as much current as it can into the battery and the voltage will drop to the battery voltage + some small amount. As the battery gets charged its chemical voltage will rise and you will move to the right in that plot.

More sophisticated chargers will regulate the output current and voltage in such way, that the depicted maximum power point of the solar panel is used (\$P_{mp} @ V_{mp}, I_{mp}\$). But from the description of the Steca Solsum 10.10F it seems it's just protecting against over-charging and discharging.