I'm having the same problem, but with a much smaller document. I can open and save the document as long as I don't make any changes to it. As soon as I make changes, I get this error message (screen cap attached), "Save failed due to an internal error. This should be reported to the development team."
I have moved the document several times between Dropbox and the internal SSD on my MacBook Air, but with the same results.
It seems to be just with this document. I removed an attached .jpg image, but no change. I created a new document and imported the styles from the problem document, but was able to save it without a problem.
I've attached a zipped file of the document.
My MacBook Air is a 13-inch, 2017 with a 2.2 GHz Dual-Core Intel Core I& processor, 8 GB of 1600 MHz DDR3 RAM and Intel HD Graphics 6000 graphics card with 1536 MB of memory. I'm running macOS Catalina 10.15.7 and Affinity Publisher ver 1.8.6 (same with Affinity Photo & Designer).
2020_Christmas_Letter.afpub.zip