After a few months of using this project it turned out that the solid state relays where not as reliable as first thought. One time the power relay stayed stuck close, but the resistors where not shorted by the other relay. This caused the resistors to get too hot (they are not calculated for continuous load).
The burned pcb:
After this I decided to build a new version of this pcb. This time with mechanical relays:
The firmware in the microcontroller stays the same.