page template – undefined structure in source code

Search all your theme and plugin files for:

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"

Disable all plugins and re-enable them step by step. This document type declaration is not part of WordPress’ core files; there is a filter active somewhere.

Update

Try to remove all filters from the affected functions. Start with wp_list_pages():

remove_all_filters( 'wp_list_pages_excludes', 99 );
remove_all_filters( 'wp_list_pages', 99 );

I still don’t believe that the doctype is nowhere to be found. That is impossible.

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