Usually this error indicates that there is an error in code you have added manually (or was added by an unofficial extension).
This error will be displayed during the creation of the search index. In order to generate the search index successfully the HTML code of the pages must be 100% valid.
Please verify all code you have manually added for errors. Or disable the search index to prevent this error message (Menu->Tools->Search Index).
Note that in WYSIWYG Web Builder 7.5 and higher, the error will also include the page name where the error occurred.