Accessing Jira 4.0 issues using the jiraissues macro in Confluence, fails with an error Error formatting macro: jiraissues: java.lang.RuntimeException: The value '<Jira Project Key>' does not exist for the field 'project'.
Re-loading the page then gives a HTTP Status 400 error in FireFox and crashes Opera.
The user in this case, did not have "Browse Project" permissions for the project from which the issues were being requested from. The username and password were passes in the jiraissues macro URL.

A more meaningful error such as "Insufficient Permissions" would help understand the cause of the problem.

Errors in the logs:

2009-10-28 10:24:10,673 ERROR [http-8080-4] [confluence.extra.jira.JiraIssuesServlet] doGet Unexpected Exception, could not retrieve JIRA issues: The value '<Project Key>' does not exist for the field 'project'.
– url: /plugins/servlet/issue-retriever | userName: test1 | referer: http://localhost:8080/display/test/jira+space
java.lang.RuntimeException: The value '<Project Key>' does not exist for the field 'project'.
at com.atlassian.confluence.extra.jira.DefaultJiraIssuesManager.retrieveXML(DefaultJiraIssuesManager.java:189)
at com.atlassian.confluence.extra.jira.JiraIssuesServlet.getResultJson(JiraIssuesServlet.java:165)
at com.atlassian.confluence.extra.jira.JiraIssuesServlet.doGet(JiraIssuesServlet.java:98)

2009-10-28 10:27:56,679 ERROR [http-8080-5] [confluence.extra.jira.JiraIssuesManager] retrieveXML Received HTTP 400 from server. Error message: Bad Request
– url: /plugins/servlet/issue-retriever | userName: test1 | referer: http://localhost:8080/display/testItest/another+jiraissues+test
2009-10-28 10:27:56,679 ERROR [http-8080-5] [confluence.extra.jira.JiraIssuesServlet] doGet Unexpected Exception, could not retrieve JIRA issues: Bad Request
– url: /plugins/servlet/issue-retriever | userName: test1 | referer: http://localhost:8080/display/testItest/another+jiraissues+test
java.lang.RuntimeException: Bad Request
at com.atlassian.confluence.extra.jira.DefaultJiraIssuesManager.retrieveXML(DefaultJiraIssuesManager.java:189)
at com.atlassian.confluence.extra.jira.JiraIssuesServlet.getResultJson(JiraIssuesServlet.java:165)
at com.atlassian.confluence.extra.jira.JiraIssuesServlet.doGet(JiraIssuesServlet.java:98)

Putting the user back into the jira-users group and re-configuring the project permissions, helps resolve the problem.