Most likely your curves need cleanup. You mentioned joining all objects after import, did you mean all segments were separate or did they come as closed curves already and you just joined them into one single object?
I would advise checking for any loose vertices, and more importantly overlapping vertices, which are the most likely to cause the artifacts similar to your screenshot. Besides that very tightly close vertices, very short segments, and mostly very sharp angles tend to cause problems with beveling.
In your case the most likely culprit is the the font objects. Check the text part of your logo, fonts are often sloppily designed or of low quality, or simply not prepared for 3D modeling and contain overlapping or very close together vertex which can cause problems.