Code mentioned is correct, but is should be inserted at line 199 (not 289 as originally mentioned) in wpforo.php, see attached screenshot

if ( ! empty( $_SERVER ) && strpos( $_SERVER, 'sitemap' ) !== false ) {
return;
}

Above correction was later updated by rank math, I will update with the details from rank math case asap.

Peter

Hello,

We were able to reproduce the error and have logged it. Please allow us some time to test and release a fix.

It should be added to the plugin in the upcoming update. You can keep an eye on the changelog and we will be mentioning this error when fixed.

For now to fix the issue on your site please add the following code on line 289 before $this->init_base_classes(); in plugins/wpforo/wpforo.php file:

if ( ! empty( $_SERVER ) && strpos( $_SERVER, 'sitemap' ) !== false ) {
    return;
}

I hope that helps. If you need any other assistance, please feel free to let us know. We are here to help.

Thank you.

RankMath has acknowledged the issue on their side, see extract from RankMath case:

Hello,

We will implement a permanent fix in our plugin in an upcoming update. The changes made in the WPForo files are a temporary solution to resolve the issue on your site.

Please don’t hesitate to get in touch in case you need our assistance with anything else.

Thanks.

What happens after you disable RankMath, does the issue remain? If no, contact with RankMath support.

Unfortunately issue does not go away using your suggestions.

Hence pls. propose next step.

Pls. find my detailed comments below.

Ad 1:                  No

Ad 2:                  Checked, same issue when deactivating Elementor

Ad 3:                  Done

Ad 4:                  Done, issue remains when only RankMath and wpforo are active

Ad 5:                  Checked – no such old wpforo template

Ad 6:                  Checked

br Peter