Err, I think I had the very same problem just a while ago...
However, my interface system didn't use panels, but a view and some entities.
The problem was the alpha blending that messed up the outlines of my text.

After setting the blending in the interface-shader manually to

AlphaBlendEnable = True;
DestBlend = InvSrcAlpha;
SrcBlend = One;

the problem was fixed.