I was having the same problem as RNDawson with the Turbotax Bussiness 2024 edition. I'm glad I found this post because this was my exact problem, but it was with the for form 8453(digital signiture).
I rescanned the form as a .jpg instead of .pdf to reduce the file size, and it transmitted no problem.
Cheers to RNDawson for finding the issue.
