Thank you for replying. I see you're the programmer who created Clean Blog (and the Timeline template for s9y), so you'll know it inside out.
It looks to me as if the entryproperties plug-in was installed on my server (because I can see it there in FTP mode as part of the whole s9y installation), but I hadn't activated it by installing and saving the plug-in from the admin interface, as you and YellowLed thought. I've just gone through the list of all the event plug-ins again and installed it properly from the interface, so I presume I could select Clean Blog again now and it would work correctly, i.e. netmirror is not at fault after all. Is that right?
Looking at the configuration options in the entryproperties plug-in, none of them are familiar, so I guess I missed installing this one at the time; I just selected the Clean Blog template from the template changer plug-in. It got displayed properly for a while before the error message came along, though, so I'm puzzled about the real reason for that appearing.
I also omitted to read up on configuring the template, which I probably ought to have done. I can see there are lots of possibilities outlined in that description, and it also mentions the need for the entryproperties plug-in that I overlooked.
One of the reasons I like Clean Blog is exactly because it is so clear -- uncluttered and "nice and tidy". At the same time, though, I'm used to a blog having sidebars, and as you can see from the way my blog's now being displayed in Bulletproof, I put some information in my sidebars that was meant to be useful to readers, such as a dictionary look-up feature, links to featured dictionaries and list of archived blog posts. These don't appear in Clean Blog's default appearance. Would it be possible to incorporate them somehow in future? In a way that fits in with the overall design? Perhaps by letting visitors click on headings to expand a sidebar that's otherwise "folded away" -- a bit like the idea of hiding and unhiding the horizontal ribbon in Word. Just an idea.