After adding a normal map, the object becomes darker

That's something I was having issues with as well. Marcel Kuhnt mentioned the solution in a different topic.

You need to select your mesh and then in the "selected mesh" menu on the left you need to click the first button. (Should be labeled "add tangent information" or something similar, I don't have my editor set to English.

Uhh, in that case I'm lost.

Maybe Marcel Kuhnt has an idea.

Edit: Take a look at the contenttool logfile, maybe there is something useful in there.

Basically, thats all correct, but you cannot add tangents to splines, because after placing a spline, the tangent information will be added automatically. This „dark“ problem should only happen in the preview...