Odd issue here. I draw a square in Inkscape, save it and the square will not open in Signcut. Any file with a square tends to not open either, just the non square items.
I tried grouping, and "union".

Hi aj
I don't see anything about the 1st file (aaaaa.svg) that should cause a problem. But in the 2nd one (Stan's....svg) 2 of the rectangles are still rectangles, and have not been converted to paths. That seems like something that could cause a problem.

I know little to nothing about SignCutPro, except that it sounds like cutter and/or plotter type of software. What I think that means is that the software needs paths to cut along. So you could try Object menu > Object to path, to convert them to paths.

If that doesn't work, then I'm stumped. I'm not sure if it was you, or someone else, to whom I recently recommended Inkscape Cutting Design? It's a nice little forum for using Inkscape with many various cuttinng/plotting software. I suspect they know a lot of helpful tips.

Or maybe someone else here will have some ideas. Did you check in the Cutter/Plotter forum here? I know there have been other questions about using Inkscape with SignCut products. You might learn something by reading those?

PS - I just looked at the other messages you've posted, to find out if I had already told you about the other forum. I found a recent message of yours that hasn't been answered yet, so I'll try and answer it, right after this. I'm not sure how I missed it, but I'm not here every single day (almost though ) -- not sure why no one else replied either. But I'll be there right after I post this

Thanks I think I tried that before, and just tried it now. Also changed the biggest rectangle to a different color. I did the Path>Object to path method. It still opens just the letters in the cutter software. Iwill try the sign cut forum and browse there.
When I do Object>Stroke to Path for the red or black rectangle it just disappears or blends with the lower rectangle. If I find answer I will post it back here.

edit:
for the two interior rectangles, red and black one, I selected a node and clicked "join two" thinking was adding a node, which it seemed to do; I was thinking maybe more nodes would make it more of an object somehow. This worked for the red n black, but not for the outer gray rectangle which I can NOT seem to be able to select a node. the red and black appear in the sign cut.
Since the red n black appear, but not the gray I deleted the gray, duplicated the black, changed color, lowered to bottom and widened to become the outline lower object. It now works. Seems like a backwards crazy way to make a simple rectangle appear in SignCut.

I can quite understand what you've done, but if you've gotten it to work, then that's the main thing.

After you did Object to Path, did you save the file again, before sending to SC? That should have worked. I'm wondering if you maybe didn't have the right things selected, at certain steps?

Well anyway, I'm glad you got it to work