Hi there – I’ve done some digging and have found out why it doesn’t work.
The reason is actually quite simple. It’s because BBPress doesn’t support shortcodes in forum posts. At first I thought this was strange but actually – once I thought about it – it did make sense. If anybody could enter shortcodes then they could enter any shortcodes – including the bbpress ones. For example a random user could enter the login form, or worse, the entire forum, into a forum post.
Unfortunately there’s no way to run a single shortcode only. WordPress will process all shortcodes or nothing. So whilst browser-shots would be safe, because others are not it’s been blocked entirely by bbpress.
Sorry about that.
I shall keep this in mind and see if there’s a way to make browser shots work – but at the moment it doesn’t look very promising.