Followup to this report. I closed the report with: Having discovered the dodgy USB port on my server (see #808879), I switched ports and now I can print to this printer remotely with no problems.
So I'm closing this bug report. But it turns out that there was a second problem at play: Mac OSX 10.11 (El Capitan) now tries to connect to CUPS servers using IPv6 wherever possible, and so I had to allow the server to accept requests from certain IPv6 addresses. More details here: http://unix.stackexchange.com/questions/235483/printing-to-linux-cups-printer-from-mac-el-capitan-fills-error-log-and-doesnt/250250#250250 Why the log floods when the connection is not allowed, though, is beyond me; perhaps the bug report should be reopened? Julian
