Is there any way to find out which e-mail I used when I registered this account ?

no

http://forums.d2jsp.org/contact.php

contact paul himself and he might help you. takes some time though, but its defenatly worth the shot.

Request an email change here:
http://forums.d2jsp.org/recovery.php

Select the first option:
"I am trying to recover my account, but I forgot or no longer have access to the registered email."

Make sure to provide as much detailed information as possible; the more, the better.
Include things that only you would know, dates of orb purchases, tracked topics, etc.
Also be clear that you're in ownership of your account and you merely are looking to change your old email address which you no longer remember.