Well if your users are setup to allow only 20 minutes to edit posts and you changed it to -1 does this not mean that they tried to edit the post before it was posted?
This is how I think
Post at 5pm
Changed setting to -1
Now 20 minutes pass.
Time is now 4:20pm
Try to edit post at 5:20 but now is 4:20pm.
Looks like you are editing the post before it was made.
Maybe try again and clear cache or wait longer to see if it is still an issue.
