subscription-manager fail: Unit with ID <guid> could not be found

Anyone recall a problem like this, from the good-old-days?

For tried-and -true RHEL 7.6 (a requirement from our customer) on a newly generated system image:

sudo subscription-manager register --user xxxx
Registering to: subscription.rhsm.redhat.com:443/subscription
Password:
HTTP error (404 - Not Found): Unit with ID "da731453-c99d-4e28-8f61-dce936c1f327" could not be found.

Subscriptions requested by user xxxx with matching password do work on other machines (e.g. tested on a VirtualBox VM.)

Here are some other outputs, in case they prove illuminating:

sudo system-manager list
... shows:
+-------------------------------------------+
Installed Product Status
+-------------------------------------------+
Product Name: Red Hat Enterprise Linux Server
Product ID: 69
Version: 7.6
Arch: x86_64
Status: Unknown
Status Details:
Starts:
Ends:

sudo system-manager remove --all

... confirms there are no active subscriptions, saying:
0 subscriptions removed from this system.

sudo subscription-manager config --list
[server]
hostname = [subscription.rhsm.redhat.com]
insecure = [0]
no_proxy = []
port = [443]
prefix = [/subscription]
proxy_hostname = []
proxy_password = []
proxy_port = []
proxy_user = []
server_timeout = [180]
ssl_verify_depth = [3]

[rhsm]
auto_enable_yum_plugins = [1]
baseurl = [https://cdn.redhat.com]
ca_cert_dir = [/etc/rhsm/ca/]
consumercertdir = [/etc/pki/consumer]
entitlementcertdir = [/etc/pki/entitlement]
full_refresh_on_yum = [0]
inotify = [1]
manage_repos = [1]
pluginconfdir = [/etc/rhsm/pluginconf.d]
plugindir = [/usr/share/rhsm-plugins]
productcertdir = [/etc/pki/product]
repo_ca_cert = /etc/rhsm/ca/redhat-uep.pem
repomd_gpg_url = []
report_package_profile = [1]

[rhsmcertd]
autoattachinterval = [1440]
certcheckinterval = [240]
splay = [1]

[logging]
default_log_level = [INFO]

[] - Default value in use

Any advice would be welcomed.