I was wondering whether anybody else had this specific issue and found a solution:
Normally I run the ASI1600MM-PRO on a Raspberry Pi and it works just fine. Due to another problem I had to use a MacBook Air. This has only USB-C ports and to connect the camera I used an Apple adapter (the one with USB-C, USB-A and VGA). With this connection Ekos cannot establish a connection to the camera anymore. In the log the message is: "Unable to establish: + ZWO CCD". To me this appears to be a problem specific to the adapter and INDI, since the following things do work: with the same cable and adapter I can connect to the camera from the laptop just fine with the ASIStudio software. Inside with the desktop computer the camera happily communicates with Ekos, both with the same cable or a USB2 cable, when plugged-in directly to a USB-A port.