A computer may have been switched off or crashed while the software was open and it has corrupted the data files.
XML data files were manually opened using another program such as Notepad, Microsoft Word, etc. and saved the file with or without modifications.
You must consider the suggestion in the order they have been listed.
If you have taken data backups from the software, restore Accounting Period data for that company. Details are provided in 'Backup and Restore' section of the software user guide and Help files.
Delete the relevant Accounting Period for the Company and then create it again. Please note you will lose all the data you have entered so far and the process is irreversible.
(Accounting Period should NOT be deleted if returns for that period have been processed and submitted, or a subsequent AP has been created)
Disclaimer: Information provided here is given without any obligations and we will not accept any claims or liabilities for any damages as a result of you relying on the information given here, if in doubt you must consult a qualified system engineer. Any web links provided to third party website are for your reference, we do not have any control on the validity and content on those websites. We do not take any responsibility for you using or relying on them. Copyright to contents remains with the owner of that website.