not sure where to start with some potential problems… just not sure any would cause this issue… but let me get those out first…
you have some embedded player plugin that is throwing js errors on every page load…
your theme is incorrectly loading jquery ui… its loading its own custom version instead of the version that comes with wp that other plugins expect…
but as I said, despite those potentially causing significant errors, I dont see it related to this since its not js related… but you should correct those too…
another thing I notice is your theme is using something called pagebuilder… there is a plugin by the same name that can cause conflicts with other wp plugins including simple press because of how things are done… but dont know if this theme just includes the pagebuilder plugin or has its own similar idea and just reused the name…
but can you check and see if the pagebuilder for the theme must be used on all wp pages? can you disable page builder stuff on only the wp page that the forum appears on? users of the plugin have been able to do this… might get rid of the need for the strict wp api and other intricacies…