I could use some help fixing or working around newly surfaced Vegas 15 problems since last editing 3 months ago:
(1) 2hr clip of 3840x2160 XAVC-S with a single synced MP3 audio track.
Dell Inspiron 3847, i3-4160, 8GB RAM
Video preview window frequently goes to black while editing.

[ NVIDIA GeForce GTX 1060 3GB card not recognized any more as a choice for GPU acceleration. Only choice is none or on board Intel HD 4400 - currently going with HD 4400. ]
*** installed CUDA Toolkit, and video card returned to the optional preference GPU acceleration list as an option***

(dunno if this ever worked, 1st time that I tried it...) "An error occurred while creating a proxy for a stream. The reason for the error could not be determined."
Have tried un/reinstalling Vegas 15

I know that comp specs are sketchy on the low end, but a few months ago I was editing (3) 2 hr clips together of the same specs without these problems.

I have seen the Nvidia card disappear under preferences in Vegas when the Intel HD graphics driver has caused an issue with Vegas seeing the Nvidia GPU. This may or may not be the case in your situation but it is worth checking out if this is the cause.

To do this normally you have to boot into BIOS and disable the Intel onboard graphics. Not knowing your PC or BIOS I can't advise any further other than to say when I have disabled Intel graphics in BIOS the Nvidia GPU selection has then reappeared in Vegas and worked correctly. Seen this now on three systems.

What can cause the Intel onboard HD graphics to cause an issue all of a sudden is if the BIOS has been updated and during that update process the Intel HD graphics selection is activated. In Win 10 the Intel HD driver is not a standard driver install. You have to download this from Intel. So if you have done any Intel chip set updates it may have installed the Intel HD graphics driver. If so you may have to uninstall it.

I think what happens is there is a clash of graphic drivers under Vegas that hasn't been resolved. In the cases I have seen this issue Vegas has had very poor playback capability which dramatically improved once the Intel HD graphics option was disabled in BIOS and the Intel driver uninstalled in the OS.

Happy hunting!

Chris Young

Thank you so much for those ideas. I'll check for those options.

~ So, turns out that the culprit is the NVIDIA GeForce GTX 1060 3GB card. As I dabbled more, focusing on the card, I realized that I'd bought/installed the card just after the last editing, and had not used it for editing, though I'd used it for everything else since.
I didn't have a BIOS option to disable the onboard graphics altogether, only an option to disable multiscreen functionality, so I disabled the Intel HD 4400 in Device Manager.
Once the NVIDIA was the only one active, the black screen issue went away, but frame rate dropped to an unusable level, even at Draft/auto. This was way worse than the preview levels of the Intel HD 4400 before the card.
So I've swapped monitor cables back to the onboard connections, re-enabled Intel HD 4400, and disabled the NVIDIA.
Money wasted on the NVIDIA [/sigh].
Thank you, Chris. It was the interaction between the two that caused the symptoms of black screen.