Air Carrier Captain reports confusion among the cabin crew over a malfunctioning jumpseat that has been MEL'ed but not properly identified to the crew.

Narrative

Read logbook via computer; awaiting my commuter flight. Noted MEL 25-02c for 4R aft door Flight Attendant jumpseat no retract. MEL states that seat must be secured retracted with 'tape of a contrasting color' and that the nearest passenger seat must be reserved for the Flight Attendant; and placarded 'For Flight Attendant Use Only'. Commuter flight was 4 hours late; and I was 1.5 hours late for sign in. Aircraft was loaded and the crew and passengers were ready to go when I boarded. [I] gave a brief to the Purser; who was someone familiar to me; and asked if they had taken note of the placarded seat. She said they had; so we departed after completing all checklists. Logbook was properly filled out and placarded for the jumpseat; but I did not physically check the seat. Two hours into the flight we got a call that the 4R jumpseat would not retract. I asked if it had been secured retracted and was told it was not. Entered in logbook that seat was not secured in accordance with MEL. Later; [I] received another call that there was some misunderstanding. There had been an issue with a passenger seat during boarding; and the Purser thought this was what I had referenced in my brief. Since I wasn't there during boarding; I was unaware of this issue and it wasn't in the logbook; and none of the Flight Attendants were aware of the placarded Flight Attendant jumpseat. Apparently; the jumpseat had been secured with tape of the same color as the seat; and the tape had come loose so the seat was available for use; but it still did not retract properly. The appropriate passenger seat had been placarded for Flight Attendant use; but the Flight Attendant MANAGER; prior to boarding; had apparently assumed it was an unauthorized action by the Flight Attendants to obtain a crew rest seat; and removed the tape and placard. Hence; we departed using a placarded and malfunctioning Flight Attendant jumpseat. Once all this became known; the Flight Attendants secured the jumpseat as best they could and used the appropriate passenger seat for landing.