This thread helps Fix error 306 & 400 while trying to run/start your Steam VR (Beta) device.
If you met an error msg "Steam VR failed to Initialized for unkown reasons. (error: Shared IPC Compositor Connect Failed (306))" upon starting Steam VR, try the following possible solution: Check your GPU drivers. To do that, Visit either AMD or NVIDIA's website and download the latest version. 1) NVIDIA drivers support page@ goo.gl/HhtDBk "NVIDIA Driver Downloads". 2) AMD drivers support page@ goo.gl/4kAgcR "Download AMD Drivers & Software for Radeon, FirePro, APU, CPU, desktops, and laptops". Plan B: Uninstalling & reinstalling your device drivers (using the latest setup file) may help. Be sure to toss in a reboot to ensure that all of the changes take effect.