I just updated my V-Ray to todays’ update and my lightcache is rendering black. What gives? Not a huge deal with progressive, since I get a preview pretty fast with BF, but with buckets, my preview is a bucket at a time.
Check your logs. There may be a warning about some feature that causes light cache to not preview (like a render mask for instance).
No, nothing. I have been rendering all day. I noticed the update text on the VFB, updated, and this is when the LC stopped showing. I’ll reboot MAX when I get a chance; maybe it’ll fix itself.
Weird. Could be a bug. I don’t know. I just know some features disable LC previews.
Boy you sure do rarely restart Max, eh? I do it every chance I get just to clear everything out. Not sure how you could update VRay with Max running, though.
I closed MAX to update.
This is still happening, and no, there is no log errors. After a Google search, it is a know issue, but no resolutions.
Don’t believe everything you read online. 8)
Send a scene to support. There is likely an easy fix. If there is not then there is a bug they need to know about so they can fix it.
Unfortunately, it’s a very large scene, but I’ll try.
A new scene, same problem. I did a material override, so it isn’t a material.
https://drive.google.com/file/d/1wlGtOTiBcWKNSLeJBUQTftpr94YVPM2Q/view?usp=share\_link
It is disabled when viewing the composite mode 
Can you elaborate?
Initially I thought it was the back to beauty itself, but actually it’s the fact that the back to beauty automatically switches from rgb to composite mode, which does not show the light cache calculating.
Well, that is a bug! Thank you!
No problem. Don’t know if it’s a bug per se, as of course the light cache is in fact calculated, but certainly it’s something that can cause confusion, as it’s not obvious at first glance 
I don’t know if it’s mentioned anywhere in the docs but probably should be noted there to save others’ headaches
Any V-Ray elements are calculated on the rendering phase, hence why the image appears empty in Composite during the LC phase. Happens without Composite mode also - just add any element, switch to it, and render.
Still, should it switch automatically to composite, from rgb, on render start?
A small point, but clearly where the confusion arose
If Back to Beauty is active, yes.
Okay, how do I turn back to RGB? I toggle it, and it switches back to Composition.
With your scene, it was missing lots of stuff so the cache calc happened too quick to switch, but it does switch if you get it before the calculation is finished, otherwise it’ll just auto-switch.
Yeah, but it goes back to composition without my input.