I have this same error. I tried optimizing my attachment....renaming my attachment to a smaller version, etc. Also hard install, and various other attempts to fix. I've efiled many other returns that had much larger attachments.

Ultimately, I called Lacerte.

They told me that they ARE aware of this issue, and have been working on it all through March and still now!

They took my info, and are supposed to email a message when the error has been resolved.

Very frustrating....but you can call them to get on their email list.