I got this error in
main.CRITICAL: Broken reference: No element found with ID 'checkout.header.wrapper'
and the checkout page is blank.
I solved this warning message in Magento 2.1.1:
Open your file (override file not in vendor)
In line equals:
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" layout="1column" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
I found this solution here.
The Broken Reference layout errors are nothing new and have been reported to Magento team numerous times by different people. I tried it on a fresh installation just to see whether it was my custom theme or changes that were causing it but with default luma theme without inheriting it I still get these errors.
I somewhat agree with @KAndy above that these errors can be ignored because according to Magento core team this was fixed in 2.1 but it's still going on and reported by numerous people. It doesn't affect the functionality of the system in any way (or it hasn't been reported by anyone yet) so it's not a priority for Magento core team hence for now it can be ignored. Of course my only concern is ever increasing size of the system.log file that will need to be cleaned up regularly other than that nothing to be overly concerned about.