MF COBOL Error 181 on OPEN - Bogus Error Code?

[Database Administrator]

I'm grasping at straws, so I figured I'd throw this out to the
Assembled Multitudes to see if anyone has any ideas. I've already
posted this in the forums at HP's ITRC, but I thought reposting over
here wouldn't hurt, either.

We're on HP-UX 11.11 running Micro Focus COBOL 4.0:

$ cob -V
version @(#)cob.c 1.313
PRN=RXCPR/AAD:9i.T4.40.04
PTI=SP2
I see no work
$ cobrun
V4.0 revision 0 build 10/10/2 G; 14705. Run Time System RXCPR/
AA0/00000D

Intermittently and unpredictably, we've got a program aborting on an
OPEN statement with the following:

I/O error : file 'RECOOUT1'
error code: 9/181 (ANS74), pc=0 call=1 seg=0
181 Invalid parameter error

The SELECT statement for the failing file is

SELECT RECOOUT-FILE ASSIGN TO RECOOUT1.

The OPEN statement is

OPEN INPUT RECOIN-FILE
OUTPUT RECOOUT-FILE

Before the program is run, we set an environment variable RECOOUT1 as

export RECOOUT1=/absolute/path/to/file

This error only seems to occur when the system is "busy" - lots of
other programs all running simultaneously, all beating up on the
system, but none of which are *supposed* to be accessing /absolute/
path/to/file. We've never seen the error occur when the system is
"calm", only when it's being hammered.

I think the 181 is bogus, since we're not directly calling anything
where we'd need to move around a "parameter". My gut tells me that
this is likely one or more kernel parameters that need to be
increased, but since I can't tell what the message is trying to
communicate to us, I can't tell which parameter(s) need to be tweaked.

Does anyone have *any* idea where I could even start with this?