If you can’t cut the plain rectangle the problem isn’t with the file, but it might have gotten hung up trying to render that PNG if you tried that one first - I’ve had it get stuck on a complex file before and not go anywhere until I completely cleared everything.
If you want to try to clear it before support pops in, log out of the app and do the whole reset starting with holding the print button until it turns teal, then let it run completely through the calibration before entering the app again. Might take a while, so have some breakfast or something while it does it’s thing.
If that works and it gets unstuck, I’m going to make a suggestion that will sound a little strange but it might work for your PNG file - try rotating the image in your outside design software by 22.5° so that the lines in it run in more of a vertical and horizontal orientation, then re-rasterize it.
I can’t turn the machine on to test this at the moment to see if it actually works, but I was thinking mostly horizontal and vertical lines in an image might be easier for the software to interpret into the horizontal engraving movement. It might interpret and load faster, and won’t get hung up. Will be a lot less data too for the buffer to handle.
The other thing that might help is to do a jig with the cutline in the file, rotated through the same 22.5°. It can handle cut lines at any angle easily.
Anyway, you probably know all this already, so if you’ve already tried the reset and it didn’t work, you might need to wait to hear from support.