troubleshoot recent global posts

I have post indexer installed and network activated and indexing posts from across my network nicely. These are displayed in the post indexer settings page. When I network activate recent global posts and insert the [recentglobalposts] shortcode into a page, my upfront theme says: "Ooops, something seems to have gone wrong with rendering the widget"
How can I find out what might be causing this and fix it?
Thanks - Mark