A321 Captain reported a malfunction of the flight deck door at cruise with the First Officer locked out of the flight deck after a bathroom break. The Flight Attendant eventually was able to release the door and the First Officer returned to the flight deck during descent and the flight landed safely.

Narrative

During cruise flight; and with less than two hours to our destination; ZZZ; the First Officer and I planned a lavatory break. First Officer went back first and when they called to come back to the cockpit; the Flight Attendant was not able to unlock the door. We tried every possible way; including the code and the bolt but unfortunately with no success. I sent several messages to Dispatch with no reply. I then called Dispatch over the phone and was put on hold for Maintenance to be online. The Dispatcher admitted that they received my message and were trying to get hold of Maintenance. When Maintenance came online; the only reply I received was that there was nothing they could do as we were approaching ZZZ. I went through the QRH and MEL but could not find any procedure to unlock the door manually. The bottom mechanical latch was stuck. I [requested priority handling] and started preparing for the approach. The Flight Attendant was a great resource during the whole process. As we descended through flight level 180; I asked the Flight Attendant to call the number-one Flight Attendant; and we tried to unlock the door for one last try; which worked. First Officer was back in the cockpit and resumed their duties as the pilot flying. I rescinded the priority handling and landed uneventfully. Cause: Old doors and not maintained well