Dog is suddenly afraid of his harness?

Is it possible that when the harness was being put on, it pinched a little bit if hair or skin, and now the pup associates it with pain?

IMPORTANT: if your dog is actually registering pain after the harness is on, especially while walking, at the very minimum, you need to check for sores and even tumors, especially under the axial, aka armpit area. dogs usually tolerate a lot more pain than humans before they start whimpering and whining so if they continue to show signs of pain after the harness is on and you don't find any sores or lumps I would take them to the vet.

It's a good idea to always close the harness with your hand under it to make sure the closures never pinch your dogs skin or pull their hair. Even having a tiny bit of hair closed it can make a dog associate it with pain.

My dog does that too! My previous dog did that as well. It felt like out of nowhere they were suddenly afraid of their harness. They would start running away from me, hiding under the table, and running away when I approached. I then realized it's a "game", they both actually like when I started chasing them and they felt like we are playing a fun game. But that game stopped being fun to me as it would take me forever to put their harness on them. And these 2 very well behaved and trained dogs would NOT come to me if I was holding the harness....so I had to start unlocking the door like I am leaving ALONE, and that moment they would approach and I would put the harness on.

Are there any fear periods that happen around that age? Has anything happened around the harness- I did it fall and make a loud noise, did someone put it on too roughly, or did it get yanked too hard accidentally? Is your pup having any shoulder pain/does the harness limit shoulder mobility? Any neck pain in the dog?

My 5 year old pitbull does this. It's a neurotic learned behaviour. And in their mind it's kind of a odd game. The reason is they want to go outside unleashed. That's all it is. And there's a way to stop it all together. You have you hang it on your elbow barely but not completely hidden and grab your personal items And just walk to the door to leave but don't say anything besides a casual "alright let's go" or whichever phrase you use. Then when your dog goes to the door to leave with you, which he will, calmly clip a lead to a collar on his neck, then step out the door , then put the harness on, d3tatch and the reattach the lead, pulling it out from under the harness if applicable. And wash rinse repeat

The reasoning for this will probably always be unknown, but for whatever reason your dog has now decided the harness is unsafe. You could either work them through that with counter conditioning, giving rewards anytime they look at or otherwise engage with the harness. Or, you could just pick up a new collar/harness and see if something new that fits a bit different is easier for them to accept.