Why does an attachment table of a feature class without the GlobalID field cause errors?

Answer

In ArcMap, creating an attachment for a feature class in a geodatabase also creates an attachment table and a relationship class. The attachment table contains the attachment files and the relationship class relates the features to the attached files.

If the feature class has a GlobalID field, the attachment table is created with a REL_GLOBALID field.

The errors occur in workflows, or when using geoprocessing tools, that require a GlobalID field if the attachment table does not have the REL_GLOBALID field. The errors still occur even if the GlobalID field is added to the table after the attachment is created, because this does not change how the attachment is created and its relationship to the feature class.

Therefore the attachment table and relationship class must be re-created after the GlobalID field is added to the feature class.