To elaborate why you get that "Clipboard empty" message.

CC3+ doesn't embed images in the drawing, but link to external files. This is why you can't simply paste from the clipboard, since the clipboard isn't a file CC3+ can refer to.

The reason it is reported as empty however, is that the Windows clipboard is kind of smart. It can keep the same data in multiple formats (for example, if you copy some text from a web page, the clipboard will store both a plain text version of the text, as well as a formatted version). Whenever you paste, the clipboard will pick the version that is suitable for the program you try to paste into. However, the only thing CC3+ can accept are either plain text, or CC3+ entities, and after copying an image from somewhere else, the clipboard won't contain any of those. Therefore it is empty as far as CC3+ sees, because it doesn't have anything to offer that program.

Are those lines 0-width lines? Try making them a bit wider, thin lines can disappear when output to an image, which again would prevent them from having any effect.

You don't seem to have the Point Light Finalize effect at the end.

Does it look like this inside CC3+, or only on the exported image.

If the latter, try this

I also noticed that there doesn't seem to be a light source located in the torch in question, while the torch north for it, as well as the one on the south wall both have 2 light sources in each. (Use Dungeon - Lights -> Show light symbols to visually see their location)

I don't think there is a way changing symbol references using macros. But as a workaround, you can just explode the symbols in the drawing. In most cases, this won't make a visual difference, but the symbols are now just a collection of primitive entities which should react nicely to color changing commands. Obviously, they no longer will be regarded as symbols for commands operating on those, but that is usually not that big a problem.