Repro:

Find a machine with recent AMD hardware, Windows 10 and current drivers. tested using AMD's November 15th 2019 driver build (26.20.13031.18002)
Download and import the attached repro project DenoiseFailBuiltIn.zip
Open the Scene Test.unity
Ensure Radeon Pro is selected as the Direct and Indirect Denoiser
Ensure Lightmap Resolution is set high enough to require more than 1x1024 lightmap
Hit Generate

Note:

GPUZ records 7.7gb of VRAM used when denoising a simple Scene with cube on plane. Seems to be a massive jump in memory usage going from 1x set of lightmaps to more. Even 1x1024 and a 1x512 is enough to cause the machine to stall.

Expected:

Ideally we should be able to denoise lightmaps regardless of Scene complexity and available VRAM. At worst, we should just fall-back to no denoising and write an informative error to the Console.

Workaround:

For AMD card users, set the maximum lightmap resolution to 1024 until this issue is fixed.