This bug is now fixed. The Jira issue is
http://www.jforum.net/jira/browse/JF-604
The new code is different that the suggestion posted in the issue page, but if you want to get it working with 2.1.7 instead of waiting for JForum 2.1.8 you can try to hack the source code yourself using that code, that should work ok too.
Rafael
[originally posted on jforum.net by Rafael Steil]