After trying Mark's suggestion of removing the DOCTYPE declaration
altogether, I ran the transformation scenario and got the exact same
error. So I believe that the error must be referring to something else,
although to what I do not know. Is it possible that it has something to do
with the stylesheets I am using? Since I transformed this EAD file
successfully using a different stylesheet before, and it is the only thing
that has changed, it seems logical to suspect them.