Just a follow-up: when expanding the details of the message there was an additional note “The given key was not present in the dictionary.” My apologies that this is not the most informative error. In the case of your library it appears that there are scenarios referencing a project that does not exist in the library. Somehow it looks like the project got deleted but the scenarios did not. Is this a library that was accessible on a network via multiple users? One possible use case leading to this problem is multiple users making edits to the library at the same time.