plugin not working

Hello
I was trying to use “phpbb_recent_topics” plugin for my wp website. I followed the instructions in the topic: https://www.ads-software.com/support/topic/compatibility-with-phpbb-3-2/, but there is still a problem. When I try to add a phpbb_recent_topics widget on my right sidebar, although it shows me the list of recent topics in the preview, it tells me “not valid value”. Now I don’t know what to do, because there is no indication on where is the problem inside the code. If it can be useful, the widget is not showing the title in the preview. I attach also a screenshot: https://www.meteoregionelazio.it/wp-content/uploads/2018/04/Image-2.jpg

WP version: WordPress 4.9.3
phpbb Forum version phpbb3.2.2: https://www.meteoregionelazio.it/forum/

thanks