This gets us out of a hole on one test system which had UID 1234 in use;
it is not a full fix as we also (try to) use 5768.
+++ /dev/null
-1234
-5678
--- /dev/null
+CALLER_UID
+5678
timezone = EDT
tls_advertise_hosts =
trusted_groups = 42
-trusted_users = ${readfile{DIR/aux-fixed/TESTNUM.trusted}{:}}
+trusted_users = ${readfile{DIR/aux-var/TESTNUM.trusted}{:}}
unknown_login = unknownlogin
unknown_username = Exim Testing
untrusted_set_sender = *
receive_timeout = 0s
sender_unqualified_hosts = localhost:some.host.name
no_tcp_nodelay
-trusted_users = 1234:5678
+trusted_users = CALLER:5678
unknown_login = unknownlogin
warn_message_file = /home/exim/test/warnmsg_file
no_syslog_pid