Ah, now I know the reason for "65". Thank You !
The following recipe is for years the first in my included user procmailrcs
|/usr/bin/formail -I "From"
This indeed causes the trouble with procmail.
I changed -I to -X, and now there is no program failure shown in
procmail.log. But the problem still keeps on existing. The mail seems to be
delivered correctly to the users mailboxes. However - there is no new mail
in the /var/spool/imap/user/*user* directory.
I didn't change any config files in this mail environment for years, so
I suppose a bug in cyrus-imapd 2.3.14-8.3 to be responsible for this.