There was a post here a short time ago about this. It seems that some of the numbers are easily mistaken for another, perhaps it is the font used. Problems like make sure what you think is a five, actually has a flat top and the eights are quite round (if I remember correctly). Use zeros.

Did it also say not to use spaces?? I can't remember, you might want to double check that.

However, there was one person who continually failed and upon calling Adobe learned that his serial number was given in error as it was appropriate for the other version of the program.