When there are multiple local signatures on a key and you then add an exportable signature. gpg --edit-key asks multiple times if you want to mark them as exportable.
The editinteractor does not handle this state and goes into General Error.
I think the expected behavior would be to promote all local signatures to exportable. Although I am not sure if this would really work that way. The use case behind multiple local / exportable signatures are Tags / Remark annotations.