Christos,
Aside from the services that you have already checked and they're running, another common cause of this error is needing to reset the DAQ device. After running an example in Traditional NI-DAQ, the board needs to be reset before running an NI-DAQmx example. In MAX, expand Devices and Interfaces. Then right-click on Traditional NI-DAQ Devices and select Reset driver for Traditional NI-DAQ. Hopefully, you will now be able to use the board with NI-DAQmx. Hope this helps. Have a great day!