Dear Rhesa,
I face the same issue and was able to get rid of this error.
Under WE20, you will see 4-5 columns for Outbound Parameters like Partner Role, Message Type, Message code, extension, etc
The first column PARTNER ROLE entry plays an important role here. Try blanking out the Partner Role and test. It shall work.
Note: you cannot directly edit the partner role. So, copy the related partner profile, blank out the partner role and save. Delete the old partner profile.
Let me know if this does not work.
Rahul