Oh man! I can only imagine what you must have thought when you got that message telling you that your phone was locked again! I think what may have happened here is when you unlocked your phone the first time, it may have been a temporary unlock. We will need to take a closer look at your account and the history of your unlock requests to really find out what happened here.
Since this is primarily a user to user community, we (the community manager team) do not have account access. I would recommend that you reach out to our social media care team using the Facebook or Twitter links in my signature as you will be able to communicate with them through private message and they can see a holistic view of your account.
My IMEI shows up clean on IMEI Status Check | See if Your Phone Works on T-Mobile 4G Network but, customer support shows it's associated with unpaid balance by a previous owner. Who has the correct and authoritative information? If the internal info is correct, why doesn't the IMEI Status Check page not also warn us?