field.attach error when trying to create a redhen_affiliation connection

field.attach throws an error when I try to add a redhen_affiliation relationship. This happens if I try to add the relationship 'affiliated with' from the contact record and if I try to add it through the organisation record 'Organisational affiliation'.

the error is thrown on modules/field/field.attach.inc line 802. Information on it is:

error - field_cardinality
message - endpoints: this field cannot hold more than 1 values.

I have reinstalled redhen and have not changed the relation type in any way.

Is this an error you have run into in the past? Do you have any insight as to what might trigger it?

Thanks.