Originally Posted by Scapevision
So at this point I get the following out of what's been done:
The scanner fails to produce anything in the green channel when doing a negative scan. Positive is fine with Epson Scan, which means it's not your hardware that is causing the issue. Unless when you select Negative as media it tells the scanner to do something specific with the hardware, which I doubt. I believe it just reads a raw data and applies the inversion and tone. To me it seems like a software issue, something somewhere is screwed. I would consider checking with a computer running a different OS version along with a proven currently working setup, basically just replacing the scanner with yours (all connections remaining from the proven one). I doubt you'll find another V800 user close-by though. Don't know what else to suggest at this point.
At least Vuescan does alter "hardware parameters" when different input types are selected, so the quoted text above is not entirely correct. Vuescan boosts green channel and blue channel (even more) when 'Color negative' is selected as media type and scanner supports individual per-channel exposure. I don't have V800, but I have scanners that produce visibly different raw files when scanned with 'Color negative' vs. 'Slide film' media type. And this is not done in software after the scan but rather through altering the exposure time for individual RGB channels during the scan.
Anyway, it sure is one of the most bizarre issues I've ever seen. Unless there was some software (that messed up color profiles) installed on all of the machines, this doesn't make any sense. I would definitely use Vuescan to troubleshoot this issue as Silverfast and Epson scan are less flexible and we don't know how the things are done under the hood.
In Vuescan verify that you have selected 'Device RGB' for 'Output color space' in 'Color' tab and 'Raw output with Scan' in 'Output' tab. Compare the resulting raw files that you get with alternating 'Color negative', 'Slide film' and 'Image' media type in 'Input' tab.
Also check what you get if you set the 'Media' as 'B/W negative', 'Bits per pixel' as '16bit' and 'Make gray from' set to 'Green'.