Had to go searching to figure out what Dropbox Transfer's extremely generic "Couldn't add some files" error might have meant and found this thread from two years ago wherein after much struggle the Transfer team was happy to point out there is a completely-unpublicized 10,000 file limit (which includes files in sub-folders selected for transfer).
This limit is not exposed anywhere in the Transfer creation UI but it definitely should be, alongside the total transfer size/individual file size limits that are already exposed.
Heck, even if the above error message mentioned that more than 10,000 files == no good it would at least be elucidating for your users who are otherwise banging their head against the wall.
Thanks!
