Thanks to all who replied,
It turns out that report viewer utility is the culprit. It allows the user
to specify a printer and then issues a file equation to check the validity
of the user's input:
file xxx;dev=zzzz
It seems that some users, while trying to direct their print outs to their
local printer, were specifying 'TERM' as their printer. Ldev 21 is the
first 'TERM' class device and thus the 'Weird condition with Ldev 21'.
Again thanks to all.
Paul H. Christidis