How to resolve error generated by XBRL Company Accounts Validator

Have run a html file though the Companies House validation tool prior to submission and it has returned the following error.
Bad Request XML
Bad Request line 1263: not well-formed (invalid toaken)
Ref: txid
Error Code 1

I am trying to find what needs to be corrected within the file for it to pass validation.
The contact centre say they are not able to help

Looking at the file in notepad - the file finishes on line 1257 and line 1263 does not exist.
I also tried with a completely different file and got the same result (error on a line 6 lines after the last line of the file)
Can anyone help please?

This forum is for the CompaniesHouse API service. The XML forum can be found
I have passed your details to our XML customer support manager.

Thank you and apologies.
It was the contact centre who gave me the details of this forum as they said they had no phone support for the problem (despite the error page giving the contact centre phone number to ring for help!)

No a problem.
I have contacted customer care for XML and they have asked if you could please post your query to that forum.
Thank you so much.