Xperia Z3 Compact Power Button Problem After Battery Replacement

Greetings! I’ve been following the article to replace the battery for my Z3C. All went well, battery was replaced. But the power button seems to have a problem: phone would not react to it.

I was able to start it by connecting it via USB and fastboot. It starts OK, calls, camera, network is working fine. The only two things that don’t is power and camera buttons. I could not give a thing about latter, but I need the former :)

Any advice what could I do? Does it smell like a mobo damage or the main flex? Inspecting the cable did not yield any visual damage.

Thank you!