It might help if you can tell us what you're creating the relationship for? For example, you said you want to link Incident to your BO and your BO to Incident, but in what format? Should there be many BO records available from Incident, or many Incidents associated to this BO? What are you naming this BO, it may be that the name is simply too long.
Worked it out, when I did the upgrade from 7.4 to 7.5 some of the user created attributes wouldn't upgrade so had to change their max length to 20 so the upgrade would work. After the upgrade i changed the values back to their original length and restored the data from a backup but didn't change the md_database_column values. Have just gone into the DB and updated those entries so they are the same (or double if unicode) as that of the table atrribute lengths (they were all nvarchar datatype). and can now create new attributes and link objects with the best of them