[nerbert]

Sometimes you can view page source and find broken html or JS by changes in font colors.

There could be a broken tag from an edit somewhere. The files showing look like they're from headinclude or headinclude_bottom. Have you edited either of them?

Both have a template hook at the bottom. It's possible that a product with an uploaded php file could be putting some bad code into one of those even with the plugins turned off. I would turn on all plugins and then go through products one by one and see if one of them is causing the problem.

Also try hovering over the little question mark icon and see it it identifies the php file the user is viewing. I suspect it's your forum index. If it identifies the page that might be a clue to which one is broken.