Uncaught exception - others.c(440) : Field conflict between null and doctype

I have an old web application that runs on neko (mod_neko with apache to be precise), where a user recently reported the following error after a page request:
`Uncaught exception - others.c(440) : Field conflict between null and doctype`
Any idea what this could be about?

Start by examining the `others.c(440)` location associated with the reported “Field conflict between null and doctype” error, then trace how the mod_neko Apache request reaches it. Try to reproduce the error with the described page request and identify the input or runtime state that triggers it; done means documenting a confirmed cause or a focused fix.

Written by the indexing model from the issue text.