Could you try completely clearing out your includes files so that we can see if there is a template, styling, or includes issue.

UBB.threads 7.5 was more forgiving of HTML issues than what v7.6 is (additionally a future release will be more strict with some templates due to newer builds of Smarty).

Isaac has a useful chart now that the includes are listed now (in the order they're used):
[Linked Image from id242.com]

You could also try testing with one of the v76 styles, you can import them from the control panel, they can be found in /install/styles

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