Do you happen to be using CloudFlare and their RocketLoader?
Are you using ForumRunner? I thought that was only updated to work with the 1.0/1.1 line.
I don't know if this may have a bearing or not
If you have RocketLoader enabled, that will be the issue... disable it.
I'd also look seriously at removing ForumRunner if it's not updated for the current code line.
And you've disabled all add-ons in the ACP and checked a default style and it still is occurring?
Without being able to see it first hand (logged in) it's rather hard to trouble shoot. The most common issue is the RocketLoader being active.