I have just entered the subscribers panel, pressed “new subscriber” inserted an email address, created the user, changed it to test subscriber and it works.
Then I opened an old subscriber, set it as “test” and saved.
If you have such error, or your database is out of synch (very strange) or there are other kind of problem.
Goto the diagnostic panel, enable log at info level and send me the files you find inside the wp-content/logs/newsletter folder if possible.
Stefano.