www programming

If I have understood your text correct, there is no need for changes if I just use the integration via page-forum.php.

I updated the plugin to 2.8.5 but the resizer doesn't work anymore.

The page forum loads all as before and nothing should be changed. Nor into phpBB added code the resizer library should be changed.
Which error you get on the browser console?

I wasn't able to access the Board because it didn't show up correctly. Instead of the whole board I just saw the header.

I restored the Backup, which I did minutes before and everything works normal. Tested it again, cleared the browser cache and the same issue appeared.

I have understood your text like "No Changes need if you use the Template Integration"
Thats the reason why I didn't edit the overall_footer like mentioned above to
Maybe thats the mistake?

Substantially, using the page-forum.php way for the template integration, you had to change nothing, nor into phpBB.
While if using the integration by shortcode (that i advice to use because it work fine into any WP template, old and new) the above need to be applied.
The result is that at moment, it is not possible anymore to use both the page-forum.php integration and the shortcode integration because they call for different iframe resize libraries.
The page-forum.php need to be updated so to use the new libraries, then will be possible again to use both the template integration via shortcode and page-forum.php together.

Than I dont use die Update. If there is no improvement for the Template-Integration. idk why but the update is definitely causing an issue with my template integration.

As soon as I update the plugin, the page-forum-php is just showing the header.