This exact thing happened to me.  I burnt the original information I received when I purchased 2.5, but no serial number.  Subsequently, upon buying the upgrade to 5.2, I didn't get sent a serial number.  I followed the directions (typing in the url with my address at the end) to generate an email with the serial number.  The email DID get generated, but it sent a very friendly "here you go" email with no actual serial number (i.e. headers, but no content).

Could it be that I bought the original so long ago you were running a different system of some sort?  Snowcovered has both my original and upgrade purchase on record if you need to confirm.

Thanks!