Besides making the trust, I have seen:

 

No JIRA Projects available in Confluence

Error

Making a JIRA Report, no projects were available, giving me this error i atlassian-jira.log:

2014-03-13 07:42:16,081 http-bio-6060-exec-47 WARN anonymous 462x2065249x1 - 77.243.49.62 /rest/api/2/project [auth.trustedapps.filter.TrustedApplicationFilterAuthenticator] Failed to login trusted application: confluence:1838577 due to: com.atlassian.security.auth.trustedapps.InvalidCertificateException: Request not allowed to access URL: /rest/api/2/project

Forencics

Examining the allowed Urls, I found:

/sr/jira.issueviews:searchrequest-rss
/si/jira.issueviews:issue-xml
/sr/jira.issueviews:searchrequest
/secure/RunPortlet
/rpc/xmlrpc
/rest/api/2/project

Solution

Thanx to: https://answers.atlassian.com/questions/55576/configuring-application-link-to-confluence-from-jira-issue-links-permission-problem

/sr/jira.issueviews:searchrequest-rss
/si/jira.issueviews:issue-xml
/sr/jira.issueviews:searchrequest
/secure/RunPortlet
/rpc/xmlrpc
/rest/api/2/project
/plugins/servlet/applinks/whoami
/plugins/servlet/streams
/rpc/soap