If you experience problems after updating to version 6.5, try to set the 'Document type' setting in Page Properties to 'None'.
WYSIWYG Web Builder uses a more strict version of HTML. However this may break manually inserted code or extensions that do not comply to the latest browser standards.
This is not a bug in the software, but browsers interprete the HTML differently in 'W3C compliance mode' then they do in 'quirks mode' (previous used by Web Builder).
If you have manually insert HTML code to your page, please make sure it's 100% W3C compliant.
Also check if the names of images, objects names and other web related values are valid (no white spaces, dots, slashes or other special characters).
All official extensions will be updated in the upcoming weeks.