I just had a report of someone who was trying to generate PDF thank you letters for recent contributions, and when they click "Make Thank-you Letters", instead of downloading a PDF they were redirected to a Contribution page on the front end.
That sounded bizarre, so I tested and it did the same for me. I can't think of any recent change on the site that could cause this. I just upgraded to the latest release (from 4.7.27 to 4.7.29) and cleared caches, but it didn't solve it.
Has anybody else seen this type of behaviour before? Where to start looking to figure out what's happening?