Required key not available

hello

dovecot -n # 2.3.4.1 (f79e8e7e4): /etc/dovecot/dovecot.conf # Pigeonhole version 0.5.4 () # OS: Linux 4.19.0-12-amd64 x86_64 Debian 10.0

what is the meaning / cause of this in mail.err:-

dovecot: imap(david)<5004>: Error: Mailbox Trash: open() failed with mbox: Required key not available

I can telnet to port 143, log in and list folders, but cannot open them.

This seems to have resulted from an experimental install (now purged) of courier-imap (I maintain a webmail program). Before this, dovecot just worked; now this problem remains even after a reinstall.

-- David Matthews mail@dmatthews.org