Licences key invalid error msg, in some cases.

Hi,

I have a client who is getting the error message, 'sorry your license key isn't valid for this domain' on attempting to log in. I, on the other hand, am not getting this error message and can log in fine. They are still on Perch v2.8.18.

Why is it an issue for them and not me?

Perch: 2.8.18
Production mode: Production (100)
Installed apps: content (2.8.18), assets (2.8.18), categories (2.8.18)
DB driver: PDO
DB tables: perch2_categories, perch2_category_counts, perch2_category_sets, perch2_content_index, perch2_content_items, perch2_content_regions, perch2_content_resources, perch2_navigation, perch2_navigation_pages, perch2_page_templates, perch2_pages, perch2_resource_log, perch2_resource_tags, perch2_resources, perch2_resources_to_tags, perch2_settings, perch2_user_privileges, perch2_user_role_privileges, perch2_user_roles, perch2_users
Users: 3
App runtimes:
<?php
    include(PERCH_PATH.'/core/apps/content/runtime.php');
?>
Editor plug-ins: markitup
H1: 5c31be2a45e2424a78eaaff5a5edef87
L1: 44168b0a6a2405c6e49c0ac70da43f15
headerColour: #ffffff
content_singlePageEdit: 1
helpURL:
siteURL: /
hideBranding: 1
content_collapseList: 1
lang: en-gb
update_2.4.6: done
latest_version: 2.8.15
on_sale_version:
headerScheme: light
content_hideNonEditableRegions: 1
dashboard: 0
content_frontend_edit: 0
logoPath: /admin/resources/1397493115_desktop-logo-invert.png
update_2.4.9: done
update_2.8.18: done
PERCH_DEVELOPMENT: 10
PERCH_STAGING: 50
PERCH_PRODUCTION: 100
PERCH_DB_USERNAME: daughter
PERCH_DB_SERVER: 213.171.200.71
PERCH_DB_DATABASE: MB_01
PERCH_DB_PREFIX: perch2_
PERCH_LOGINPATH: /admin
PERCH_TZ: UTC
PERCH_EMAIL_FROM: studio@daughter.is
PERCH_EMAIL_FROM_NAME: Daughter
PERCH_PATH: /home/linweb30/m/missionbayltd.com/user/htdocs/admin
PERCH_CORE: /home/linweb30/m/missionbayltd.com/user/htdocs/admin/core
PERCH_RESFILEPATH: /home/linweb30/m/missionbayltd.com/user/htdocs/admin/resources
PERCH_RESPATH: /admin/resources
PERCH_HTML5: 1
PERCH_RUNWAY:
PERCH_ERROR_MODE: DIE
PERCH_DATE_LONG: %d %B %Y
PERCH_DATE_SHORT: %d %b %Y
PERCH_TIME_SHORT: %H:%M
PERCH_TIME_LONG: %H:%M:%S
PERCH_DEBUG:
PERCH_PREVIEW_ARG: preview
PERCH_TEMPLATE_PATH: /home/linweb30/m/missionbayltd.com/user/htdocs/admin/templates
PERCH_DEFAULT_DOC: index.php
PERCH_DEFAULT_EXT: .php
PERCH_PRODUCTION_MODE: 100
PERCH_RWD:
PERCH_HTML_ENTITIES:
PERCH_SSL:
PERCH_STRIPSLASHES:
PERCH_PROGRESSIVE_FLUSH: 1
PERCH_AUTH_PLUGIN:
PERCH_DB_CHARSET: utf8
PERCH_DB_PORT:
PERCH_DB_SOCKET:
PERCH_SESSION_TIMEOUT_MINS: 20
HOSTING SETTINGS

PHP: 5.6.31
Zend: 2.6.0
OS: Linux
SAPI: cgi-fcgi
Safe mode: not detected
MySQL client: 5.5.52-MariaDB
MySQL server: 5.1.73-log
Extensions: Core, date, ereg, libxml, openssl, pcre, sqlite3, zlib, bz2, calendar, ctype, curl, hash, fileinfo, filter, ftp, gettext, gmp, SPL, iconv, intl, pcntl, session, standard, posix, readline, Reflection, Phar, shmop, SimpleXML, sockets, mbstring, sysvmsg, sysvsem, sysvshm, tokenizer, wddx, xml, cgi-fcgi, bcmath, dba, dom, gd, imap, json, ldap, exif, mcrypt, mssql, mysql, mysqli, odbc, PDO, pdo_dblib, pdo_mysql, PDO_ODBC, pdo_pgsql, pdo_sqlite, pgsql, pspell, snmp, soap, tidy, xmlreader, xmlrpc, xmlwriter, xsl, zip, mhash, ionCube Loader
GD: Yes
ImageMagick: No
PHP max upload size: 20M
PHP max form post size: 20M
PHP memory limit: 64M
Total max uploadable file size: 20M
Resource folder writeable: Yes
Session timeout: 24 minutes
Native JSON: Yes
Filter functions: Yes
Transliteration functions: Yes
PATH: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin
PWD: /var/www/fcgi
PHP_FCGI_MAX_REQUESTS: 100
ORIG_SCRIPT_NAME: /fcgi-bin/php56-cgi
ORIG_PATH_TRANSLATED: /home/linweb30/m/missionbayltd.com/user/htdocs/admin/core/settings/diagnostics/index.php
ORIG_PATH_INFO: /admin/core/settings/diagnostics/index.php
ORIG_SCRIPT_FILENAME: /var/www/fcgi/php56-cgi
SCRIPT_NAME: /admin/core/settings/diagnostics/index.php
REQUEST_URI: /admin/core/settings/diagnostics/?extended
QUERY_STRING: extended
REQUEST_METHOD: GET
SERVER_PROTOCOL: HTTP/1.0
GATEWAY_INTERFACE: CGI/1.1
REDIRECT_URL: /admin/core/settings/diagnostics/index.php
REDIRECT_QUERY_STRING: extended
REMOTE_PORT: 58650
SCRIPT_FILENAME: /home/linweb30/m/missionbayltd.com/user/htdocs/admin/core/settings/diagnostics/index.php
SERVER_ADMIN: support@fasthosts.com
CONTEXT_DOCUMENT_ROOT: /var/www/fcgi/
CONTEXT_PREFIX: /fcgi-bin/
REQUEST_SCHEME: http
DOCUMENT_ROOT: /home/linweb30/m/missionbayltd.com/user/htdocs
REMOTE_ADDR: 82.0.223.27
SERVER_PORT: 80
SERVER_ADDR: 127.0.0.1
SERVER_NAME: missionbayltd.com
SERVER_SOFTWARE: Apache
HTTP_COOKIE: cmsa=1; PHPSESSID=klt2c5u37mecq38vn4eekppb56
HTTP_ACCEPT_LANGUAGE: en-US,en;q=0.8
HTTP_ACCEPT_ENCODING: gzip, deflate
HTTP_REFERER: https://missionbayltd.com/admin/core/settings/diagnostics/
HTTP_ACCEPT: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8
HTTP_USER_AGENT: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.113 Safari/537.36
HTTP_UPGRADE_INSECURE_REQUESTS: 1
HTTP_CONNECTION: close
HTTP_X_FORWARDED_FOR: 82.0.223.27
HTTP_X_REAL_IP: 82.0.223.27
HTTP_HOST: missionbayltd.com
UNIQUE_ID: Wbkl17OlnO5h@O4ORm4r@wAAAAU
on: 127.0.0.1
REDIRECT_STATUS: 200
REDIRECT_HANDLER: application/x-httpd-php56
REDIRECT_UNIQUE_ID: Wbkl17OlnO5h@O4ORm4r@wAAAAU
REDIRECT_on: 127.0.0.1
FCGI_ROLE: RESPONDER
PHP_SELF: /admin/core/settings/diagnostics/index.php
REQUEST_TIME_FLOAT: 1505306071.3582
REQUEST_TIME: 1505306071

Cheers.