Yes, 2017 Build 459.

Return to Factor Settings didn't fix it either.

It's a bit strange, I can't any examples in the forums or online of anyone else having this error.

I'm on Win 10 64-bit. I have the main /bb/ folder on my D: drive, Real Tracks and Real Drums are on my M: drive.
Not sure if that would change anything.

Incidentally, when I'm in DAW mode and try to drag and drop, the cursor immediately switches to the 'no go' circle with a slash icon.
And since I get the same error whether I'm tring to drag 'n' drop to a DAW or simply to explorer, it must be some setting I'm overlooking in BIAB, although I'm stumped as to what it could be.

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PC: Win10 Pro 64
32GB RAM + Two 250GB SSD + Two 4TB Externals
Software: BIAB 2017 / Reaper / Sonar / EZDrummer 2 /
EastWest Composer Cloud / Amplitube 4
Guitar: Gibson SG T Heritage Cherry