www programming

How you do not have access to your forum?
It is impossible. Last update was only about the template integration.
The plugin do not change nothing into phpBB. Nor the extension whenever you use it.
So how you cannot access to your forum due to these?

I don't know.

What I do know is that as soon as I installed update 2.8.5, I no longer had access to my forum.

I have this displayed when clicking on a link in my menu :

probably because you did NOT added the name of the page where the shortcode has been added, into the related integration plugin admin where there is this option:
The (awesome) iframe template integration using shortcode
so the code that should process the shortcode, return what you mention, it is not processed at all.

If you added the shortcode into a page named myforum
you have to add myforum into the field of the option
so that when this page load, all the necessary to process the shortcode will be added. If not, the shortcode do NOT fire.

and the above output, indicate that you are adding it via something not correct. ARe you adding the shortcode via the WP default shortcode widget or how?

it should be:

Yet yes.

See attached image.

how you add the shortcode on page forum

and why you use Forum instead of forum as setting here?

I don't understand the first question.

I put forum instead of Forum, but it still doesn't work.