Completely new to Blender and ProRender and testing to see if GPU renders work. I can render Blender test files with Cycles/Eevee but when I use ProRender I either get a transparent frame (see attached) or Blender crashes after a few minutes.
Also, if I render that default opening scene (cube), ProRender is much slower than Eevee even though on a my Mac, Eevee has no GPU support. Does ProRender work with existing Blender materials and lights or do you have to convert them?
Have tried every combo of CPU and GPU, the render status from the bottom states 100% and I'm viewing the rendered result, slot 1 combined - see attached incase I'm not using Blender right
Hope that all makes sense and sorry if I'm using Blender wrong, very new.
File: Wasp Bot from: Demo Files — blender.org
Mac Pro 6,1 12C, 64GB RAM, D700s
Blender 2.80 (2.80 2019-07-29
Crashed Thread: 0 Dispatch queue: com.apple.main-thread
Exception Type: EXC_CRASH (SIGILL)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY
Termination Signal: Illegal instruction: 4
Termination Reason: Namespace SIGNAL, Code 0x4
Terminating Process: Blender 
I'm having more or less the same problem. On two different systems (an iMac and a MacBook pro both with Mojave installed). The MacBook Pro has an Intel GPU and a quad core i5 so Prorender shows only the cpu for rendering. On the iMac, both the dual core i5 and the Radeon Pro 555 (Metal) can be used. With these settings, Prorender only works on the iMac but terribly slow (Cycles takes 4:44 minutes to render a test scene and Prorender ~20 minutes). Disabling the Radeon on the iMac makes Prorender behave as on the MacBook Pro: no render at all, only a transparent frame (the same as OP's provender.jpg screenshot)
Is there some way to have a look at some logs?
Blender version is 2.80
Prorender version is 2.0.112
I made some more tests. Disabling the PBR Denoiser made ProRender work with my initial test scene.
I know this is not a high end machine... anyway I hope this will be useful for someone. I run two tests with the startup Cube scene using ProRender default settings except Tile Rendering is activated (64x64). CPU+GPU is much slower than using CPU only...
This are my machine's CPU and GPU and default ProRender settings:
Cube CPU+GPU, 3:11 minutes:
Cube CPU only, ~57 seconds: