The sun is shining through the walls, even though I have bFloatPointRenderTarget=1.

You don't have bFloatPointRenderTarget=1 set, at least not in the correct file.

Check whether you have multiple SkyrimPrefs.ini files on your system, for example certain mod managers create their own copy.

_________________
Embers HD
Smoking Torches and Candles
Particle and Subsurface Scattering Patches for ENB (updated 17/1/2020)
flickr

I've checked, I only have one SkyrimPrefs.ini.

Have you done a search on your disks? The correct one without mod managers is in My Games/Skyrim. If you have additional copies for example in the game folder, remove them.

Countless people have struggled with the same problem, and as far as I remember, on every single occasion they were editing a wrong file.

_________________
Embers HD
Smoking Torches and Candles
Particle and Subsurface Scattering Patches for ENB (updated 17/1/2020)
flickr

Nope, just the one in My Games.

Same issue here and it's driving me nuts that I can't fix it.

Based on my poor observations it's somehow related to the graphics card/driver.
I can get it to work when I set ForceFakeVideocard to true.

I'm currently downloading the last 3 drivers before the newest beta (320.00) which are:

Setup:
- SkyRealism Evolved ENB Cinematic version 0.139
- ENB 0.166

320.00 Beta - Sun Rays not working
314.22 WHQL - confirmed working
314.07 WHQL - confirmed working
310.90 WHQL - confirmed working

I'm still working on the new beta driver but so far it might seem as it's a graphic driver install hiccup. I'll do another run on the new beta driver tomorrow but for now it seems it's causing troubles.
If the problem is not FPRT=1 related I'd suggest:

1. Perform clean install of current driver (nVidia installer supports this)
2. Roll back at least one major driver release (eg. 314 -> 310) and start upgrading from there until you hit it.

Thanks, that worked.