Order edit page broken

Question

For few orders, the order admin page is broken.

For other orders, page is working fine and normal without any issues.
XML ERROR while turning on php errors

Warning: simplexml_load_string(): Entity: line 16: parser error : XML declaration allowed only at the start of the document  in /var/www/html/lib/Varien/Simplexml/Config.php on line 510

#0 [internal function]: mageCoreErrorHandler(2, 'simplexml_load_...', '/var/www/html/l...', 510, Array)
#1 /var/www/html/lib/Varien/Simplexml/Config.php(510): simplexml_load_string('loadString('loadFile('/var/www/html/a...')
#4 /var/www/html/app/code/core/Mage/Core/Model/App.php(380): Mage_Core_Model_Config->loadBase()
#5 /var/www/html/app/code/core/Mage/Core/Model/App.php(293): Mage_Core_Model_App->_initBaseConfig()
#6 /var/www/html/app/code/core/Mage/Core/Model/App.php(337): Mage_Core_Model_App->baseInit(Array)
#7 /var/www/html/app/Mage.php(684): Mage_Core_Model_App->run(Array)
#8 /var/www/html/index.php(94): Mage::run('', 'store')
#9 {main}

Answer 1

In your xml file

First line start with

<?xml version="1.0" encoding="UTF-8"?>

No comment above it or no any space before it and Do not start with new line.

Not like below:

<!-- validators -->
<?xml version="1.0"?>

Answer 2

The issue was due to some third party extension, when I disabled the third party extension from app/etc/modules the page appears fine.
Then I wrote to the extension support team and they provided me the fix.