clarry: Hmm, our software is similar enough that I suspect this could be an issue with the game trying to use a display mode that your monitor does not support. (I'm also using RX480 with open drivers, MESA 18.3.6, kernel 5.0.9-200)
I could not figure out a simple way to stop the game from mode switching. It's possible to disable fullscreen mode but the game still does a mode switch at startup before reverting back to the desktop resolution.
I tried running the game in a virtual desktop in case that was an issue, but it did not help. The game seems to think there is no hardware acceleration available - this is what sof.log shows after enabling logging:
------- Loading ref_gl.dll -------
ref_gl version: GL 0.01
Initializing OpenGL display
...setting mode 3: 640 480 FS
...attempting fullscreen
...using gl_bitdepth of 32
...calling CDS: ok
...registering window class: ok
GLimp_Init() - no hardware acceleration detected
VID_CreateWindow() - GLimp_InitGL failed
ref_gl::R_SetMode() - invalid mode
ref_gl::R_Init() - could not R_SetMode()
ref_gl::R_Init() - could not load "opengl"
Initializing OpenGL display
...setting mode 3: 640 480 FS
...attempting fullscreen
...using gl_bitdepth of 32
...calling CDS: ok
...registering window class: ok
GLimp_Init() - no hardware acceleration detected
VID_CreateWindow() - GLimp_InitGL failed
ref_gl::R_SetMode() - invalid mode
ref_gl::R_Init() - could not R_SetMode()
ref_gl::R_Init() - could not load "3dfxvgl"
ref_gl::R_Init() - could not load "3dfxogl"
Cmd_RemoveCommand: avi not added
R_Shutdown() - Shutting down
************************************************************
ERROR: Could not initialize OpenGL rendering environment!