[cups] Connecting to Sharp LPR printserver

[Michael Below]

Hi,

I'm using cups 1.7.5-11 on Debian testing, which works fine with a
number of different printers, local and networked. 

Now I am trying to print to a Sharp MX-3110N networked copier/printer
with a builtin LPR printserver via local network. From Windows, I can
use this printer as a LPR printer, using the IP address and spool name
"lp". Using the same settings in CUPS doesn't work. CUPS waits
permanently, saying "Connection refused" and "The printer is in use",
while it can be used from Windows PCs at the same time. In the debug
logs, it looks like this:

D [31/Jul/2015:15:32:26 +0200] cupsd is not idle any more, canceling
shutdown.
D [31/Jul/2015:15:32:27 +0200] cupsd is not idle any more, canceling
shutdown.
D [31/Jul/2015:15:32:34 +0200] [Job 86] Connection error: Connection
refused
W [31/Jul/2015:15:32:34 +0200] [Job 86] The printer is in use.
D [31/Jul/2015:15:32:34 +0200] cupsd is not idle any more, canceling
shutdown.
D [31/Jul/2015:15:32:35 +0200] cupsd is not idle any more, canceling
shutdown.
D [31/Jul/2015:15:32:49 +0200] [Job 86] Connection error: Connection
refused
W [31/Jul/2015:15:32:49 +0200] [Job 86] The printer is in use.
D [31/Jul/2015:15:32:49 +0200] Report: clients=7
D [31/Jul/2015:15:32:49 +0200] Report: jobs=37
D [31/Jul/2015:15:32:49 +0200] Report: jobs-active=1
D [31/Jul/2015:15:32:49 +0200] Report: printers=5
D [31/Jul/2015:15:32:49 +0200] Report: stringpool-string-count=39331
D [31/Jul/2015:15:32:49 +0200] Report: stringpool-alloc-bytes=21160
D [31/Jul/2015:15:32:49 +0200] Report: stringpool-total-bytes=721600
D [31/Jul/2015:15:32:49 +0200] cupsd is not idle any more, canceling
shutdown.
D [31/Jul/2015:15:32:50 +0200] cupsd is not idle any more, canceling
shutdown.


According to "sysctl status cups", cupsd is running fine:

 cups.service - CUPS Printing Service
   Loaded: loaded (/lib/systemd/system/cups.service; enabled)
   Active: active (running) since Fr 2015-07-31 12:10:19 CEST; 3h 3min
ago
     Docs: man:cupsd(8)
           man:cupsd.conf(5)
 Main PID: 1717 (cupsd)
   CGroup: /system.slice/cups.service
           ├─ 1717 /usr/sbin/cupsd -f
           ├─ 3667 /usr/lib/cups/notifier/dbus dbus://
           └─12709 lpd://192.168.3.30/lp 85 root Selbsttestseite drucken
1 jo... 

lpstat works fine locally, and I can ping the printer, but if I try to
get status information from the LPR printserver, lpstat throws an error:

$ LC_ALL=C lpstat -h 192.168.3.30
lpstat: Bad file descriptor

Any tips how to proceed? When I google for the "bad file descriptor"
error, I get results for CUPS issues about properly configuring the
local CUPS server so lpstat can connect. But in my case the local CUPS
config works, on local and remote printers, and lpstat connects fine to
it. It just doesn't want to connect to an external LPR printserver. And
I want to know why. What's the proper tool to test that?

Any hints are welcome, I don't really know how to debug this.

Cheers
Michael