Cant update (or change) firmware: URI invalid. URI empty

So, i have this arducopter 2.8 sitting around for 2 years by now, never used it since my last quad (and only used for testing, after that I changed it to a PIX), and 3 days ago, i gave it a try on my new plane, but when I was going to update firmware (using MissionPlanner), this error popped.

Since then im trying to fix it, I never used this forum before (I don’t even know if this post is in the right place) but I know that its directly related to APM boards, and after many researches on the forum I didntfound anyone with the same problem.

It worked 2 years ago, but dont wont to work now.

PS. All the lights on the blink board just right, and after measuring the power outputs it seens right, Im realy lost now.