Please ensure that all required fields in form 5a are correctly filled out. The error message indicates that the 'Field other activity' section may have an issue. Double-check this section for any missing or incorrect information. Make sure to provide accurate details related to the 'other activity' as required. If the error persists, consider reaching out to the relevant authority or IT support for further assistance. It's crucial to resolve this issue promptly to proceed with registering for the employer DSC successfully.