So everyone understands it:
someone hacked the forum and got access to the file system.
They changed some files and included an iframe.
The iframe loaded the new blackhole exploit kit v2.
And this loaded (on my machine where I saw this) some payload, it loaded also a jar file.
Google chrome blocked it for me directly.
The php error was due to the changed files and just fooled you. There was more than only this error message, the iframe, but you could not see it.