I think I found out what the problem is: the camera doesn't work well on a USB2 port on Linux (pardon the sarcasm but we did hear this before with ZWO, didn't we?). I found this out tonight when I plugged the camera in to one of the USB2 ports on my raspberry pi 4. When I took an image, it was completely black with one white column, exactly as I saw a couple of weeks ago when I thought that the driver was bad. When I disconnected the USB cable and the power from the camera, reconnected the power and plugged it into a USB3 port, it worked again and it kept on working all night long.
The mini-pcs that I used have USB ports that in principle work as USB3 ports but may switch to USB2. So I think that in some cases it may have immediately switched to USB2 when the camera got plugged in, resulting in black images and image download issues. And in some cases it would start out as USB3 and then switched to USB2, resulting in Ekos reporting both exposure times for the ASI6200 camera.
I have reported this to ZWO and asked them to check and verify my theory. If they can confirm it then hopefully they can fix it in a firmware update. In any case, from now on I'll make sure to only use USB3 ports for this camera.
Wouter van Reeven
ASI6200MM and 7 slot 2" filter wheel with a SkyWatcher Esprit 80 ED on a SkyWatcher HEQ5-Pro
ASI1600MM-Pro Cooled and 5 slot 1.25" filter wheel with an 8" TS Ritchey-Chrétien on a SkyWatcher EQ6-R