[cups] empty page_log file

[Spike]

Dear all,

i'm running Ubuntu Server 14.04 with cups 1.7.2. I have configured two
printers, HP M401dne and HP M225dw. Both printers are working, but in some
cases I get a "filter failed" and no logs for me to look at for debugging
as both page_log and error_log are empty.

# cupsd.log
...
LogLevel debug
PageLogFormat %p %u %j %T %P %C %{job-billing} %{job-originating-host-name}
%{job-name} %{media} %{sides}

#cups-files.conf
...
ErrorLog /var/log/cups/error_log
PageLog /var/log/cups/page_log

I'm using the ppds from hpliprinting.org, latest
3.16.5, hp-laserjet_400_m401dne-ps.ppd
and hp-laserjet_pro_mfp_m225_m226-ps.ppd . they both use hpps as a backend:

administrator at printer-srv:~$ egrep -i '^\*nickname|^\*cupsfilter'
/etc/cups/ppd/*
/etc/cups/ppd/M401dne.ppd:*NickName: "HP LaserJet 400 M401dne Postscript
(recommended)"
/etc/cups/ppd/M401dne.ppd:*cupsFilter: "application/vnd.cups-postscript 0
hpps"
/etc/cups/ppd/M225dw.ppd:*NickName: "HP LaserJet Pro MFP M225-M226
Postscript (recommended)"
/etc/cups/ppd/M225dw.ppd:*cupsFilter: "application/vnd.cups-postscript 0
hpps"

any idea what might be wrong?

thank you,

Spike