Well the dialogue box there comes from your browser informing you of a conflict between your site - which appears to be https and your forum setup that perhaps is not. So the only question is - how was the website built for https and why doesn't WordPress know that?
i would start by checking the two url's you have set up in the WP admin > Settings > general and ensure they are both set to https. These WP core settings are what WP uses to inform plugins such as SP of the scheme being used.
What does 'Can’t edit post either' actually mean? Describe please.