I had this problem too, with a KiCad file. It worked with a previous Proteus file so I compared the two. It’s to do with how Mayhew Labs’ gerber view parses the dimensions.
It appears it expects a leading zero (thou cords?), rather than the default KiCad exports, which is decimal format:
Decimal format: X4.9606Y-3.6575 (buggers up board size and drill location as described)
Leading zeros: X049606Y-036575 (works fine)
So the fix is to write leading zeros if your CAD package has the option. If the developer of 3d Gerber Viewer is looking at this, I imagine it would be an easy fix for the parser too.