This can happen if there are any additional new lines inserted in the cobranding.config file before the actual <xml> declaration tag. The below line should be the very first line in the cobranding.config file followed by the rest of the xml tags:
<?xml version="1.0" encoding="utf-8"?>
Once the cobranding.config file has been corrected to ensure no new lines or white space before the <xml> declaration tag above, the co-branding package will need to be re-built and deployed again on the TIBCO Spotfire Server. After that corrected package has been deployed, the TIBCO Spotfire Server can be started.