No USB network connection ( when booting from eMMC

I successfully operate the BBB with the latest SD-card image (Debian 8.4 2016-05-13 4GB SD LXQT) when booting from SD-card.
The BBB shows up as and its accessible via ssh.

After successfully flashing the eMMC on the BBB (uncomment the last line in /boot/uEnv.txt) and boot from eMMC, the BBB does not show up as I cannot access the BBB via USB/ssh.
The heartbeat is blinking as expected.
The flashing procedure seems to be successfull, because the LEDs were blinking as documented in the update tutorials.

What do I do wrong ?

How can I debug ( serial conneciton is not available right now) ?

I’ve found the USB Ethernet Gadget to be somewhat unreliable in the 2016-05-13 images. Often I need to press the reset button and reboot two or three times before it works. I think there was some race condition that may be worse booting from the eMMC.

If that doesn’t work, it won’t hurt anything to boot the SD card again and let it write the flash again.

For watching the boot via a serial console you need a USB to serial dongle:

This page should get you started if it need/want to do this