@RogerS
The fix happened when I downloaded and installed a new driver from Intel for the ARC 770 (it wasn't linked yet in their update routine, but was on their site).
The RAW decoder, as you note, is probably NOT involved in the problem or the solution, but is mere coincidence. It happens that the AUTO choice for VP21 is to use integrated graphics for RAW and "external" graphics (card) for Decoder in the File I/O tab. Mine had been set to ARC 770 for both, but that was probably not the problem; it was the driver.
Why Build 187 exposed an Intel driver incompatibility that Build 108 didn't have...that remains a mystery.
Thanks, all who helped address this peculiar but crippling issue.
Thank you for noting that the new driver fixed the issue- that should help others who encounter this problem and hopefully find this thread.
Glad you are able to work with VP 21.187 now.
@IAM4UK glad you are up and running! It amazes me how drivers can be problematic. Bouncing around between drivers is time consuming and a little frustrating trying to find the 'right one'. Even looking at the AUTO choice when you have more than one graphics option can be a source of things not working. But I, like you, have had AMD driver issues between builds and versions, trying them to find the "right one".
I've wondered why, with a consumer editor I sometimes use, graphics driver issues were never thing...
@Howard-Vigorita Did you see any similar issues as the OP here? You also have an 11th gen Intel K CPU with Arc 770?
@RogerS I use my Arc a770 as the main Vegas gpu (selected in video prefs and with hdmi plugged into it). But only for multicam edit mode when all cameras need to be on-screen at once. I've been using the 101.4644 driver for my recent projects. It's not only solid but smoother than my higher powered 6900xt gpu doing that before color grading or video fx. However, I keep the 11900k's igpu disabled in bios when using Vegas. Having 2 different Intel devices present at the same time may be causing Vegas some confusion. Intel Hyper is intended for setups like that which Vegas has not implemented yet. I only enable the igpu in bios when using apps that support Hyper, like Resolve.