This issue can often be seen in the var/log/system.log when an xml document has some issues.
2018-03-17T02:50:01+00:00 ERR (3): Warning: simplexml_load_string(): Entity: line 2: parser error : XML declaration allowed only at the start of the document in /var/www/vhosts/example.com/httpdocs/lib/Varien/Simplexml/Config.php on line 510 2018-03-17T02:50:01+00:00 ERR (3): Warning: simplexml_load_string(): in /var/www/vhosts/example.com/httpdocs/lib/Varien/Simplexml/Config.php on line 510 2018-03-17T02:50:01+00:00 ERR (3): Warning: simplexml_load_string(): ^ in /var/www/vhosts/example.com/httpdocs/lib/Varien/Simplexml/Config.php on line 510
You will need to find the cause of the problem and rectify. It is often caused by xml files that are invalid, or information that is missing in the xml files. You could use a process of elimination and disable each extension individually, or use an xml validation tool to validate your extension (or layout) files. Via SSH, run the following from within your Magento installation:
xmllint --noout app/etc/modules/*.xml
or
xmllint --noout app/design/frontend/yourpackage/yourtheme/*.xml
This will identify any invalid files and indicate where the issue lies:
[user@magevps httpdocs]# xmllint --noout app/etc/modules/*.xml
app/etc/modules/Example_Extension.xml:2: parser error : XML declaration allowed only at the start of the document
<!--?xml version="1.0"?-->
^
If you are a Create Hosting customer on the Magento Hosting Plan, feel free to submit a support ticket and we may be able to assist you with this.
