You MUST provide the column name in ForeignJoinColumn attribute (only thing you can do is omit the ForeignJoinColumn attribute as a whole, if you are using [Automapping] attribute).
ok, I reverted my code to the one with that informed and it is now back working again.
foreign key. The referenced column must be unique in the associated table. This
parameter is optional, if it's not specified (and usually it won't), the name of
Id field will be used - in other words, the
primary key of the associated table will be referenced by the foreign key."
The documentation says that about the >>ReferencedColumnName<< parameter, which is the third parameter in ForeignJoinColumn attribute, not the >>Name<< parameter, which is the first one. I think it's very clear.