I myself just tested the “git submodule update” command. It seemed to work. Is there a chance that you do not have an internet connection or that a connection to assembla.com could be blocked by something (e.g. firewall, etc.). There is another reason this could happen, if you have an old version of git or are running an older software that does not support the newer security protocols required by the connection, it will give you an error like the one you are experiencing.Try cloning the openadc repository outside of the chipwhisperer install manually. This should give you a more intelligible error message.
Note: the OpenADC submodule is not required unless you want to update the firmware on the FPGA. This is not required for most users as the FPGA come pre-programmed.
Finally, the only reason I can think of that the GUI would come up blank is if you are running CWMainGUI.py instead of CWCapture.py. Since the CWMainGUI.py is the parent class of both the capture and analyzer gui it mostly just contains the shared structure of the GUIs (Mostly a blank window).
Also make sure, if you are manually running the ChipWhisperer gui, execute
cd <path to directory containing chipwhisperer>/chipwhisperer/software && python CWCapture.pyw
where will have to be changed to where chipwhisperer is located on your computer.
I hope this helps!