There are two things that I can confirm that cause this to happen. One is in the code the other has to do with the user's browser.

1) If I am in the process of deleting posts in a thread then the link on the page can reference a query string that is higher than the number of posts for that thread. Since the code does not check for that error you get a blank page. (it is on the list to fix)

2) Sometimes browsers will append additional data to a query string usually but not always due to a improperly coded browser add-in or helper or due to malware on the users PC. If the addition data is numeric and integer based then the referenced page will be blank due to the reason above. I have looked in the logs and seen many improperly returned URL/Query string from IPs that browse the forum.