As a test, I set up JIRA 3.12.1 and Confluence 2.7, and attempted to tell JIRA to trust Confluence. This is the error I got:

For the purposes of this bug, i don't care why it isn't working. I care that:

• The error message presented to the user is completely unhelpful.
• There is absolutely nothing in the logs.

JIRA code is swallowing a NPE, which ought to be a hanging offense.

If I try the http URL instead of https, I get an equally uninformative message:

It's Confluence 2.7 which does indeed support the Trusted Application protocol. Again, a complete lack of information necessary to aid further debugging.