Another forum that I use to test wpForo displays the same issue.

I will contact pro-support and provide URL as advised. Thanks @blackraz 

There is no alternative. Your forum is private, and we don't expect you to disclose your domain and login password publicly. Instead, you just need to request our Pro Support Team via email and send the private URL. This information will be visible only to some members of our Pro Team, so there's no need to worry about privacy.

The website URL is required to allow our developers to inspect the HTML elements on your website. This will help us find the problematic element and use CSS code to disable it only for mobile devices.

You can request assistance from our Pro Support team and provide private information about your website. Our Pro Support team will help you.

I could do that but not until our forum goes public, which may take years. For now, this option is not possible.

Is there any other way?? 

Although I'm no programmer, I could try to understand CSS code, our forum (wpForo 2.2.7) has been already customized a bit to meet our needs.

There is a conflict with the WordPress theme. From your screenshots, we can see that the WordPress theme menu is visible on the mobile version. You can fix that using the CSS code. For more help, we require your website URL.

I'm using wpForo in our forum under the WP theme "Holyday Plus" which worked fine until last wpForo 2.2.7 (or maybe earlier?).

When someone tries to post a new topic on their smartphones AND they open the editor to "full screen", suddenly there appears a black intruder over the editing window: the forum main menu.

http://wpforo.com/wp-content/uploads/wpforo/default_attachments/1702258442-smartphone-format-conflict.jpg

I can replicate the issue on a laptop by reducing browser tab-window width to anything below 750px.

- Is there a way to avoid this issue?

Thanks.