'Invalid account name or registration key' error, when reached max of 20 computers pending approval, should be clearer

Agreed, this is "as designed". But, the error message could indeed be clearer.

This bug was fixed in the package landscape-client - 18.01-0ubuntu1

---------------
landscape-client (18.01-0ubuntu1) bionic; urgency=medium

  * New upstream release 18.01:
    - Ported to python3 (LP: #1577850)
    - move Replaces/Breaks landscape-client-ui rules to landscape-common
      (LP: #1560424)
    - Add a libpam-systemd Depends if built for xenial (LP: #1590838)
    - Some units not reporting swift usage (LP: #1588404)
    - Fix missing install directories for landscape-common and drop
      usr/share/landscape as its only used and created by landscape-client.
      (LP: #1680842)
    - Fix VM detection for Xen, by returning "xen" only for paravirtualized and
      HVM hosts, not for dom0. (LP: #1601818)
    - Add an indication of truncation to process output that has been truncated
      prior to delivery to the server. (LP: #1629000)
    - add /snap/bin to the PATH when executing scripts. (LP: #1635634)
    - Save the original sources.list file when a repository profile is
      associated with a computer and restore it when the profile is removed.
      (LP: #1607529)
    - Drop the legacy HAService plugin, which is no longer used.
    - Avoid double-decoding package descriptions in build_skeleton_apt, which
      causes an error with Xenial python-apt. (LP: #1655395)
    - Remove dead dbus code and textmessage (confirmed not supported in server
      for ~2 years). (LP: #1657372)
    - Move bzr-builddeb conf file from deprecated location to debian/
      (LP: #1658796)
    - Support for new server error message about there being too many pending
      computers already (LP: #1662530)
    - Add a timestamp to the package reporter result (LP: #1674252)
    - Check if ubuntu-release-upgrader is running before apt-update (LP: #1699179)
    - Implicitly trust file-local sources managed by landscape. On upgrades,
      add the trusted flag to the landscape file-local apt source file if it's
      not there. (LP: #1736576)
    - Use local system tools to change the user's password (LP: #1743558)
  * clean up packaging and getting in sync with the new landscape version:
    - d/rules: drop extra:suggests which is unused since 13.07.1-0ubuntu2
    - Remove antique postinst code. No supported landscape-client version
      installs cronjobs anymore (since a long time).
    - d/landscape-client.docs: the README file is now a markdown file, so
      install that instead.
    - d/landscape-common.postinst: no need to single out
      /var/lib/landscape/.gnupg when fixing ownerships, just do it over
      the entire parent directory.
    - guard user and group removal via an empty .cleanup.* file in post, so we
      only remove the user/group if we were the ones who created them at
      install time.
    - lintian: remove absolute path from update-motd calls in maintainer
      scripts
    - d/rules: drop special handling for dapper, hardy and lucid, which are no
      longer supported.
    - d/rules: make sure we have an "extra:Depends=" in substvars even if it's
      empty
    - d/rules: drop dh_pycentral handling, it's obsolete
  * Dropped (already included in this version):
    - d/p/set-vm-info-to-kvm-for-aws-C5-instances.patch:
  ...