After you eliminate the possibility of a cable or driver issue, check out camera power.
The ASI2600Mx cameras are among those that need more power than can be supplied by the USB connection alone. Normally, the extra power needed to run the camera and read out the sensor is supplemented by the power connection to the TEC cooler circuit.
Are you running the TEC cooler and if so at what power level and temperature? You report that camera is getting 13 volt power but it may be current starved if the source of that power is not enough to run both the TEC cooler and also share power with the rest of the electronics. If the 13 volt power source cannot source enough current to run both at the same time, the internal voltage in the camera will drop and you can lose the USB connection.
Even if you do not use the TEC cooler, you must supply power to the power cable connection on the camera. ZWO advises of this issue on their Website. A similar issue can happen on the ASI6200Mx and ASI294Mx cameras.
If the issue remains after checking USB cables and drivers, try a different power supply or simply turn the TEC cooler off to see if that frees up enough current to eliminate the short-exposures-only problem. If you can shoot longer exposures with the TEC turned off but the TEC power connected and powered, then you will need to find a better power source for the camera.
Doug, I note that you participated in a similar thread back during the summer. Is this thread something new and different?
Check out the following link to the ZWO Website regarding this issue.
There are also several threads here on CN for this issue. Here is of them.
Edited by jdupton, 01 December 2021 - 07:36 PM.