Problems

The following appears in the atlassian-confluence.log:

com.atlassian.confluence.extra.jira.TrustedAppsException: USER_UNKNOWN;	Unknown User: {0};	["abc@abc.com"]
	at com.atlassian.confluence.extra.jira.DefaultJiraIssuesManager$1.handle(DefaultJiraIssuesManager.java:112)

Diagnosis

• Application link with Trusted Application Authentication is configured between Confluence and JIRA. 
• Both applications are using Crowd for user management. 
• Using an alias in Confluence.

Cause

The username from the cwd_user table in Confluence is the aliased name, which is used to authenticate against JIRA through Trusted Application authentication.  However, the requirement for Trusted Application authentication is that the username from both applications match. 

Resolution

Use OAuth Authentication for the application link.