[exim] Temporary internal error

Hello,

I'm trying to bring my new Exim config to life but am not successful.

The version is 4.93 on Ubuntu 20.04. The command 'exim -bV' doesn't show
me any obvious errors. (It does show a lot but I don't understand all of
it.) Yet when I try this test command, I get errors:

---
# exim -bt demo@???
demo@??? cannot be resolved at this time: Temporary internal error

# exim -bt test@???
test@??? cannot be resolved at this time: domains check lookup or
other defer
---

The address demo@??? is registered in my database as a valid
mailbox. And gmx.de is a valid external domain. This sort of command
used to work, but now it looks like there's a problem.

But what is the problem? Journal, mainlog and paniclog have nothing
about this event. Where should I look for the relevant problem
description? How should I know how to proceed?

---
# exim -bV
Exim version 4.93 #3 built 14-May-2020 13:29:45
Copyright (c) University of Cambridge, 1995 - 2018
(c) The Exim Maintainers and contributors in ACKNOWLEDGMENTS file, 2007
- 2018
Berkeley DB: Berkeley DB 5.3.28: (September 9, 2013)
Support for: crypteq iconv() IPv6 PAM Perl Expand_dlfunc GnuTLS
move_frozen_messages Content_Scanning DANE DKIM DNSSEC Event I18N OCSP
PRDR PROXY SOCKS TCP_Fast_Open
Lookups (built-in): lsearch wildlsearch nwildlsearch iplsearch cdb dbm
dbmjz dbmnz dnsdb dsearch ldap ldapdn ldapm mysql nis nis0 passwd pgsql
sqlite
Authenticators: cram_md5 cyrus_sasl dovecot plaintext spa tls
Routers: accept dnslookup ipliteral iplookup manualroute queryprogram
redirect
Transports: appendfile/maildir/mailstore/mbx autoreply lmtp pipe smtp
Malware: f-protd f-prot6d drweb fsecure sophie clamd avast sock cmdline
Fixed never_users: 0
Configure owner: 0:0
Size of off_t: 8
Configuration file search path is
/etc/exim4/exim4.conf:/var/lib/exim4/config.autogenerated
Configuration file is /etc/exim4/exim4.conf
---

-Yves