Got the same transmission error!
I read all messages there carefully and figured out that the error was due to a large number of crypto transactions from Betterment (~2600 transactions). Actually, TurboTax online shows summary and number of transactions for each imported form.
I followed "Summary" approach - I deleted the original form and then added manually a single transaction which I called "Mix of crypto assets" with aggregated proceeds and cost basis. After that it worked!
