This is caused when the data entered for the contacts is not complete, for example if the 'sellers solicitor' contact details are missing DX location however DX number was inserted. These fields either both need to be filled in or both need to be empty. To resolve this issue, follow the steps below:
Update the information by double clicking on contact name field and clicking on Maintain.
Add the DX location, or remove the DX Number.
Save the entry.
Save, Exit (Door Icon) and re-enter the case.
You should then be able to submit the SDLT successfully.