A somewhat bizare update...
One of our volounteer uploaders noticed that the only sub-albums which caused this issue had the word "Union" in the title (and, by default, in the filename). "Grand Union Canal", "Shropshire Union Canal" etc.
As an experiment, they changed both Title and Filename by dropping the last letter; "Grand Unio Canal" etc. And it worked!
As a "tidier" workaround, we've restored the Title (which doesn't seem to cause an issue) and changed the filename to "Grand U-n-i-o-n Canal" which seems to work as expected.
Is it faintly possible that the filename is not adequatly escaped and is throwing a MySQL error?