Thanks for reporting the bug. Unfortunately I was unable to reproduce it, however I have released a new version of the addon which I believe may have solved the issue. Could you test it out and let me know if the problem persists?
New Feature
You can now choose to have the newly registered user automatically post a thread upon registration - thanks to @Earth2Luke for the suggestion. Simply check the New User to Post Thread? checkbox. Note: You may want to change the thread message and title.
Bug fixes
Fixed...
The New User New Threads addon creates a new topic (in a forum of your choice) each time a new user is registered.
Features
Choose the forum the thread should be posted in
Choose the user who posts the thread
Have the newly registered user to post the thread themselves (automatically)
Change...