Changing opencl-index to 0 and restarting the client also helped me
I got a WU right away.
I thought I put the gpu-index and cuda to 0 as well but I see them as -1 after the client restart (opencl is 0).
Longer version: I also had "FS01:Failed to start core: OpenCL device matching slot 1 not found, try setting 'opencl-index' manually".
What didn't work: updating to the latest Nvidia driver (I had the second to last).
Have not tired: restarting the PC. I have to much work open on my computer to restart.
Windows 10
AMD 1700
RTX 2080
Latest driver & FAH client.
The fix about changing the Index I think just makes FAH use CUDA instead of OpenCL. I'm 100% sure it was using CUDA. When I tried to reset the index to -1 it said:
"WU00:FS01:Failed to start core: Option 'opencl-index' has no default and is not set."
Edit: Reinstalling FAH (+ deleting data) doesn't help. GPU-Z says OpenCL is installed. I think I'll just continue with CUDA until the next GPU driver update. If that doesn't work, I'll try DDU and reinstall the drivers from scratch.