Error found with QSH session, reason code 4,errno 3489: System resources not available to complete request

We've just recently upgraded to v5r2 and received the following message
upon an attempt to run a QSH command (FWIW, to send an email with an
attachment).

  Error found with QSH session, reason code 4, errno 3489 (from a
QSH0002 message)

Checking the archives, I found the link:
  http://archive.midrange.com/java400-l/200106/msg00051.html

But no one really explains *WHY* "System resources not available to
complete request", or how to avoid it in the future.  The message itself
has no second-level explanation.

Can anyone explain what's going on?  Also, I noticed that there's a
Retry option for this message.  It's too late for this go-round, as
someone already cancelled the job, but would a Retry after waiting a
minute or two be worthwhile?

Thanks!
Dan Bale