@crumble and @Paul Thornton
Switching on 'show hidden devices' in the Device manager (Windows 7) did not reveal anything new. Only COM1, Dell COM3 and Dell COM5 showed up (see screenprint in earlier post).
What finally worked to solve my problem is to install the driver file pycom.inf downloaded from https://github.com/pycom/pycom-micropython-sigfox/blob/master/docs/pycom_esp32/tutorial/includes/downloads/pycom.inf from within the device manager (Action >> Add Legacy Hardware >> Search for and install the hardware automatically >> Ports (COM & LPT) >> Have disk >> Browse to pycom.inf file). I installed the 'Expansion 3' option. That installed the expansion board 3 driver, but it showed an error in the device manager (device cannot start). But when I next connected an expansion board V3.1 (with lopy4) to the laptop with usb, a new COM port ('Expansion 3 COM 19') appeared in the device manager, and this one allows me to connect from Atom. Also an Expansion board V2.1A with a lopy can now be connected.
So problem solved. Although I still do not know what caused the connection problem. Auto connect does still not work because the working driver is at the bottom of the serial devices list, and the auto-connect takes the first one in the list, that one does not work. But anyway, PROBLEM SOLVED, I can connect manually by selecting the right COM port. Thanks for all the help!