I am having a very similar problem to Helmut. I am running Wishlist Member (version 2.5.786, most current version) as well as Wordpress 3.0.4, Aweber and Paypal.
Right now, when I try to process a transaction, the transaction goes through in paypal, the user is subscribed to the Aweber mailing list, confirmation emails are sent to the user and my admin account, however when it comes to the email where the registration url is supposed to be I get:
"Dear Graeme
Please Visit the following URL to complete your registration:
Error
Thank You"
Several months ago, this process was working fine but now that I am re-opening my membership site, it appears to be broken.
I checked out the ipn_handle_debug file and it had success for everything, however the subscription_handle_debug reads:
"SUCCESS :Member signup URL :Error
[01/10/2011 3:03 AM] - SUCCESS :Member signup email successfully sent using wp mail system"
I've gone through the Wishlist integration instructions several times and can't spot what I'm doing wrong... please help!