A Payload error can relate to invalid characters in text content.
To resolve this error:
Log in to HandiTax and locate your client.
Select the tax return.
From the outline view, enter the Cover page.
Check the Email address and Mobile number for accuracy.
If entered manually, delete the mobile number and email address in the Cover page and re-enter the correct data.
If you prefer HandiTax to prefill this data from your Client Details window, close out of the return.
Go to the Options menu, then Tax Form Options.
Select Email address and Mobile number to allow prefilling into the Cover page.
Log back into the tax return to review the changes made.