Well, according to this post
at Robert's Anime Corner Blog, the problem apparently is "a defective disk #3." I could still do with a bit more specificity -- defective how? -- but at least it's a partial answer.
Has anybody watched disc 3 yet who might know what the issue is? Someone uptopic mentioned a report of a video glitch in the last episode on that disc. If that's the problem, I'd like to know exactly where it occurs and how bad it is.
EDIT: Per a very nice and prompt response from FUNi to an email inquiry I sent, the problem is apparently with the video in episode 18 on disc 3 (which I don't think is in fact the last ep on the disc, despite the report cited above).