Like the CAPTCHA bug, the 10 chat dialogue bug is also a completely random issue. On some threads I can edit posts, but on other threads like this one it’s not possible.
This is on Windows 10 64 bit, using Chrome with no plugins outside of AdBlockPlus (which is disabled for this site).
My guess is that this relates to the restricted permissions of new members. After you have 10 posts work through the moderation queue, it’s not likely that you’ll run into this issue.