The following solved the issue:

EXEC sp_changemergearticle

@publication = N'mypub',

@article = N

I'm having the same issue now with the Custom business logic handler on SQL2005:

2007-09-24 15:38:25.090 Category:NULL

Source:  Merge Process(Web Sync Server)

Number:  -2146762496

Message: No signature was present in the subject.

2007-09-24 15:38:25.090...