scene darkens when interpolating 3 textures

hallo there
im trying to build a landscape in opengl and therefor i want to apply multiple textures to it.
when i interpolate between 2 textures, everything works fine.
but when i try to interpolate between 3 textures, the whole scene becomes pretty dark, and i have no idea whats the reason for this.

to fix this, ive alleady searched the internet and found an example, which does exactly what i want. ( ).
so i copied the source from the example into my own project. but nothing changed. the scene is still darker then it should. the strange thing is, that the precompiled version, that comes along with the example, works fine. it appears like it should.

here is a pic, that shows the dark scene:
and here another, with normal brightness:
and here is the part of the source, if someone wants to look at it: