I've set the options for the registered usergroups to allow editing but they can't edit threads. The time for editing threads is set to unlimited. Am I missing something?
That's correct. I've done test permissions on a few members that were only in the registered member group. All the forum node's are set up for inherit and edit posts is set to Allow.
Can you confirm when you are testing the permissions that you are attempting to edit your own posts?
As the permissions are applied to you, if a permission grants you access to do something only to your own posts (such as editing), you can only edit posts that you made, not posts made by the test user.
Had the same issue, turns out that the edit time was set to 0 in the main user group permissions for one of my user groups. Not sure how it got that way, but I was scratching my head for a good while there before finally figuring out what the problem was.