Headphones won't charge nor turn on - battery already replaced

Hi all,

I bought used Headphones and fixed them myself. Now after 3 years of usage without problems, my beats began to act weird. Some weeks ago when I wanted to use them, they went out of power. Discharging itself while turned off. So I charged it and it works for some hours. Some days later again, empty without using them. Since last week no sign of life. I bought a new battery, replaced the old one, plugged the charging usb cable but no charge indicator. I tried the 2 min's reset button combo 3 times, but nothing happened.

I checked all power sources and cables. Everything else charges fine, but not my beats...

I checked all solderings and cables but they are fine too. Is maybe my motherboard dead?

I also checked the USB Port on the beats. No sign of damage, or wobbly cable. Cable sits nice and tight.

Usually a new battery is not completely empty. It should turn on right after plugging the battery, but nothing.. any idea?