FWIW As previously mentioned, we've (luckily) never had the core licensing issue that many others have. That
might be because; We've never bothered with the XC5 free licence release scenario and / or we carry out everything (mods / own changes / customisation etc) on our server, not locally. Its hard to be absolutely sure with XC5...
In terms of the latest XC5 update, we didn't see XC 5.4.1.18 at Merchant Wave, it progressed from XC 5.4.1.17 straight to XC 5.4.1.19. No problems (for us) for a change(!) with this XC5 upgrade, but worth noting for this specific thread, that there's changes to the files shown below, which
might 
be relevant:
Code:
/skins/admin/modules_manager/activate_key/body.twig
/skins/admin/modules_manager/activate_key_v2/activate_key.twig
/skins/admin/trial_notice_v2/activate_key.twig
/skins/admin/trial_notice_v2/body.twig
/skins/admin/trial_notice/body.twig
/skins/customer/trial_notice_v2/activate_key.twig
/skins/customer/trial_notice_v2/body.twig
/skins/customer/trial_notice/body.twig