I just tried logging in from a dummy client account I created several years ago and I am getting the same error message when I attempt to upload a document..  So this appears to be an issue impacting everyone -- an inability to upload any documents.  However, if I am responding to a client template sent by the accountant, I do have the ability to upload documents in response to specific questions.

Is Intuit working on this problem?  Tax season starts on Monday and clients want to upload their documents this weekend!