We're using 2 configuration settings that come into conflict with each other:

1) we use the property "jira.permission.comment.user -> denied" on the workflow step "Closed"
2) we use the "Comment Required Validator" on the workflow transition "Close"

The post functions of the "Close" transition are as follows:
1) The Resolution of the issue will be set to Fixed.
2) Set issue status to the linked status of the destination workflow step.
3) Assign the issue to the reporter.
4) Add a comment to an issue if one is entered during a transition.
5) Update change history for an issue and store the issue in the database.
6) Re-index an issue to keep indexes in sync with the database.
7) Fire a Issue Closed Customer Canceled event that can be processed by the listeners.

As you can see, the issue is closed (2) before the comment is added (4).
This results into an error message because of the property setting in the "Closed" step (jira.permission.comment.user denied).

Unfortunately, JIRA does not allow to change the sequence of these post functions.