To investigate this I've done the following:
- I checked the form-handler and put in a completely different site page to return if a submitted form hasn’t been filled in properly. That worked fine, so not a form-handling problem.
- I tried changing the template for the error page to a very simple one I know works. Still got 1and1 404 page.
- I tried renaming the error page to ‘errant’. That still gave the 1and1 404 page!
- Then I deleted the page altogether, reset cache and made another page named ‘error’. Same problem.
Interestingly, when I tried the Preview tab while editing contents of 'error' the page displayed OK.
I have solved the problem by creating a new page called ’problem’ to do the same job. That works fine.
What's happening here? I'm tempted to say it's a 1and1 server issue (it's their 404 page after all), except the same page worked fine when I was running on 1.4.1. I see also that the release notes for 1.8.1 say that "we have eliminated the old 404 page processing method": there is no longer a 404 page tab in Global Settings (though my old 404 page works if I ask for a page which doesn't exist).
So, two queries. One is to check that 1.8.1 still has a 404 error page facility, and if so, how to find it in Admin. The second is curiosity (problem is sort of solved anyway), and that's why the name 'error' (as in www.cife.org.uk/error) has produced this odd behaviour.