When I added a trusted hostname in the Piwik settings and hit safe, the following error message came up five times:
WARNING: /is/htdocs/[path to Piwik]/core/Url.php(252): Warning - preg_match(): Unknown modifier ‘t’ - Piwik 2.13.1 - Please report this message in the Piwik forums: http://forum.piwik.org (please do a search first as it might have been reported already)
The hostname I added is an ssl proxy with the following format: ssl.something.tld/piwiksubdomain.domain.tld/
Happy to provide more details to track down this bug. Just ask.
Edit: Just noticed that the error messages comes up multiple times when I load any Piwik page, including when using domains other than the hostname in question.