Yes! You were absolutely right!
I ticked ten folders (i.e. previousely copied CD's) at a time for updating to the iPad and, when ITunes crashed, I "unticked" the folder in question and restarted iTunes and the update. I found a total of five incompatible folders, so I shall have to be happy to listen to them on the iPhone, since they don't seem to cause any trouble there...
Thank you very much to all of you helpful people!