affiliate registers click on membersite but signup on main blog

I have a bit a problem with affiliate signups. The clicks get recorded and displayed correctly on the member's site. the signups however are credited to the main blog.

tried different versions of the affiliate plugin (from 1.2.2. up to 2.4.4) and all show the same behavior.

Would be very grateful for a quick hint were to look for the cause or how to fix this.

(config wp 3.2.1. bp 1.2.9, multi-db 3.1.1)