Please see: https://simple-press.com/documentation/codex/faq/troubleshooting/i-am-seeing-multiple-forums-displayed/
Yes it helped solve the problem. Thanks.
Maybe you will be interested that the problem disappeared by itself if I disable the plugin WordPress SEO by Yoast 
Don’t know they’re related, but the forum displays correctly after turning off this plugin.
It may be WordPress SEO in combination with something else. While we do have lots of users with the Yoast plugin installed it does, sadly, appear to cause an extraordinary number of issues.
actually, cant throw WordPress SEO under the bus here… that is our own doing in to make the forum display as fast as it can be… its due to how we handle the_content filter… can cause problems but its faster and more pleasurable rendering experience for the user… but we have provided options for this type situation…