Error "The object invoked has disconnected from its clients" when calling DotNET COM dll

Hi all,
We've got a vb.net assembly (dll), which is exposing a COM interface
so it can be called from our legacy VB6 application.

When we call a particular method (which is designed to return
true/false when finished), the method works fine (it is meant to store
data in a database, and this part works), but we get the error:

"The object invoked has disconnected from its clients" triggered in
the VB6 calling application.

Is it a problem with the registration of the assembly for COM?

We've tried un/registering using regasm, but to no avail.

Any pointers please?

Thanks,

James