[exim] Sudden onset spool errors

Hi

I have *just this afternoon* started to suffer from "Failed to create
spool file" errors (Permission denied).

These errors ONLY occur when the email is submitted from Apache via PHP.
Submission via SMTP works fine, as does a direct submission from the
command line.

My Exim installation has been running successfully for several months.
The most recent detail change was over a month ago.

Notwithstanding that I've double checked all of the permissions. They're
fine of course else Exim wouldn't work at all.

Disk space is totally fine.

I'm stumped about where to start looking because as I say this was a
stable and happy Exim installation and I cannot pinpoint anything that
has changed. No system configuration work *at all* has happened for some
time (weeks) anywhere else on the server.

Suggestions about where to start debugging this one will be very
gratefully received.

Kind regards

Phil Jordan