UPDATE: This issue has been resolved, I had elided a character in editing a custom theme. Read on only if you want to be exposed to my idiocy, and Sven’s helpfulness!
I see a couple other people have reported similar things, which in some cases were diagnosed as falsely flagged errors, but this has stopped my site loading.
Any suggestions welcome. It started after I uploaded of a PNG, but I can’t see how that would be related (there wasn’t a backlash in the file name, for instance).
I can take a look if you provide a link to the custom theme’s source code (or paste it here). As I wrote above, the error message by itself is a bit cryptic.