"DataLink could not be loaded and may have been removed. Check the log for details."

I've recently tried to create a chart based on datalinks, which normally works fine, but this particular time I get a warning message in red in the chart edit page that the datalink could not be found and my have been removed. However the datalinks are there, the data connection is there and verifies successfully.

I've also checked the site.xml file and the datalinks appear in the saved file normally. But for some reason the chart edit page has an issue. It says check the log for details but I'm not sure where to check that.

thanks