Hello,
Can you tell us how many workflows and deliveries you have associated to the Campaign that you're trying to open? Campaign does have a limit (around 30 if I recall) so if you have more than that associated to a single Campaign you might start seeing erratic behavior.
Additionally can you provide some background around when this first started, if any recent changes were made, what build you are on, and if you get any additional information other than the error stated in the subject line?
Also are you using a custom template or an out of the box template? If Custom, can you try creating an out of the box campaign and delivery and seeing if the issue persists? We've had reports of this occurring in the past when custom templates were created incorrectly.
Regards,
Craig