I ran into the same issue.
Two things I think helped me overcome this issue...
1. The Account ID I fetched from the 1099-B statement had 000 prefixed to the account id. I used the exact account ID (with 000 prefixed), and that worked fine.
Earlier I was using the account id which shows up on their website (without 000), and I think that was preventing the import.
2. The document ID I fetched as-is from the 1099-B pdf statement...(with spaces in it)
Thanks
