I also had a problem with WebInstaller, it flashed, wrote twice to phone, restarted phone and then the computer stopped "seeing" the phone and interrupted the installation:
Failed to execute 'open' on 'USBDevice': Access denied.
I hadn't memory problems, tried on another computer, changing browsers...nothing. It didn't work either using the
flash-all.bat file which in theory does the same but without the browser.
But It fell into the same; when phone restart it was no longer visible.
The only option left was to get my hands dirty and type the commands by hand one-by-one.
If it doesn't help you I hope will gives you some idea: