I had a similar problem using titebond in the cold garage back in January. After I glued the strips and left the garage the temp fell below the working temperature causing white lines between the joints. I figured it would sand out later but like you experienced it did not. I used a hot iron ( the wife's spare) and went over the white lines which seem to reactivate the glue and the lines disappeared. Be careful as it will burn the wood if you let the iron sit in one spot too long. Good luck!
stev