TX6500 Malfunctioning

[Dat_Drink_Ik_Op]

I have a mayflash tx6500 who is currently halfway broken.
The problem is that half of the buttons are not functioning/reacting to my steps.
It occured after a 7 months of non use, before this it was working fine.
The nonreacting buttons are the left, up and upperleft (b) button.

I somewhat tried to open it (as you can see on the picture ) but i dont have the guts to actually 'break' it further loose cause i dont really know what 'm doing...

Anybody has an idea what the problem could be and/or how to fix it?
Now i love a challenge and when half of your steps are not registering it shure makes it hardcore to get a new highscore but its a bit out of my league :<..