Got the same missing GUI problem on Debian Stretch 32bit with OpenBox & LXSession.
Tried both ALSA & Jack version, latest one, as well as 8.52.
Managed to get GUI only once, with 8.52. Audio processing is running, Web interface is accessible.
It even shows CPU consumption by GUI
. There is no error in console, just "Opening X display.".
Had "missing GUI" in Ubuntu 16.04, too, but, there it was more like "missing GUI sometimes", so it was usually possible to get it back after 1-2 restarts...