I have no idea what causes the phobic stage but the ones who went through it the most with it lasting the longest were the youngest in each clutch although Scully hatched out a few hours after Mulder on the same day and with one clutch it only lasted about 2 days with the youngest. I do think there is a genetic predisposition for this. I've posted this before in some other thread way back when but I'll post this again because I thought it was funny and weird. Claudé, like her clutch sister Sophie, started talking at a very young age but when the phobic thing hit she stopped talking. In fact she wanted nothing to do with me and wouldn't come out of the cage on her own. About 3 years after this started I was taking care of the birds and Claudé said "Come here, give me a kiss." I was totally shocked. I opened the cage door, reached out to her and she stepped up without hesitation. Another surprise. Then she bit me quite hard and started laughing. I was in pain but very happy that she was willing to interact with me again. There was no going back from that point. She likes to come out, steps up easily, enjoys interacting with me and is chatty. However she can be nippy but I'll trade the phobic stage for some nippiness any time. Sophie never went through a phobic stage. She has always been sweet and friendly. They were curious and precocious babies who loved playing and interacting and then one of them became shy and very frightened. Both were raised exactly the same way so my only explanation for this is genetic predisposition. Maybe birth order and interaction between clutch mates also plays a role?? Raised Mulder and Scully the same but Scully was severely phobic, the 3 boys raised the same but Zack became severely phobic, etc.
I do hope that Mango comes out of it. I would remove the cover from the cage during the day if I were you and see what happens. It's just a thought. Mango does not seem to be phobic, just a bit shy and standoffish. Phobic is quite severe where a bird will generally just stay in one place and stare at you, won't take any food from you at all and won't even come out of the cage. Any new thing seems to scare him/her. It can last a long time. If I showed you videos of Scully, Claudé and Zack during that time of their lives and videos of how they are now you wouldn't even think they were the same birds.
Since most of my birds like the OFBs I use those. I keep trying with my Dusky conure and Senegal but no go. They do get other foods besides seed mix, though.
Another thing to try might be some of the Avian Organics products which you can purchase from Things for Wings or Avian Organics. My birds (except for picky Dusky) like the baked treats.