Attachment Lightbox/Slider doesn't work on mobile

This is not the wpForo Advanced Attachments slider-lightbox. Some other lightbox plugin is here, it disabled the original addon lightbox. You should do whether:

1. 1. Disable the plugin which adds this lighbox(this may be other lightbox plugins or lightbox function of non-lightbox plugin, such us Image Gallery or so...)
2. 2. Or, try to disable wpForo Adavnced Attachments lightbox, it may help this lightbox work correctly. you can disable wpForo Advanced Attachments lightbox/slider in Dashboard > Forums > Settings > Addons > Advanced Attachments admin page.

Also put this CSS code in Dashboard > Forums > Settings > Styles > Custom CSS Code textarea to fix widget avatar sizes:

.wpforo-widget-content img.avatar{
   width:48px!important;
  height:48px!important;
}

Then delete all caches and do Ctrl+F5 on forum front.