qmail unable to chdir to Maildir after Debian upgrade to 'buster'

Hi, Folks -

Looks like the ORNL qmail list archive no longer exists.  If there's
another list archive that might have this question already answered,
please let me know where that might be.

I have a qmail+vpopmail server that's been running perfectly for many
years.  Note that qmail was manually installed, not from a Debioan
package.  Since a very recent upgrade by Debian that moved release
'buster' to 'stable', incoming mail can't be delivered, generates the
error message that qmail now is unable to change to the Maildirs that are
several levels below /home/vpopmail.  All file permissions appear
unchanged.  Any idea how to fix this?  Thanks kindly.