FRM-18108: Failed to load the following objects when the object is actually loaded

When I open a certain .fmb in the Forms Builder, both in 11g and 12c, I get the message:

FRM-18108: Failed to load the following objects:

Source Module: mskt_source.olb

  Source Object: CODE_TB_TYPE

Only one object is listed, an object group from an object library that is in my Forms directory and also open in the Forms Builder.

The object group appears in the Object Navigator, and if I click on the Subclass Information property in its property sheet, I see the name of the object library and the object group without generating an error message.

I tried deleting the object group and reattaching it from the object library, saving the form and closing it, but I still get the error message when I open the form again.