I tested them one by one (exchanged with my Piezography ink cartridges. These were the two that caused the error message. They are filled with PiezoFlush and primed as the the others.
From Epson I got this explanation of the problem:
I see the message Service Error 1404 on the control panel. What’s wrong?
This error indicates that the air pressure for an ink cartridge is not correct, which is usually caused by a defective ink cartridge. If you recently replaced an ink cartridge, try replacing it with a new one. If the error still appears, contact Epson for help.