Symptoms

Adding a post-function to a workflow step fails with:

java.lang.IllegalArgumentException: Invalid complete key specified: com.atlassian.jira.plugin.system.workflow%3Aisuseringroup-condition

Cause

When adding a workflow post condition, the browser issues a POST request to JIRA, containing, amongst other things, the parameter:
type=com.atlassian.jira.plugin.system.workflow%3Aisuseringroup-condition

JIRA decodes this URL to:
type=com.atlassian.jira.plugin.system.workflow:isuseringroup-condition

In this case, there is a configuration in the Proxy Server environment that is either preventing it from being decoded, or more likely is double encoding it. Typically this is due to a reverse proxy server that is converting from SSL externally to non-SSL internally, or an SSL load balancer.

Resolution

Investigate the configuration of the proxy server or load balancer. The configuration of these devices falls outside support provided by Atlassian.