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If you have enabled readonly access to the underlying Postgres tables, then you should be able to find this id within "select * from public._users where id = 15787". It's possible that if you deleted users out of your Tableau Server, then you may need to reassign the thumbnail user that was chosen upon publishing the workbook.
I did search and the results were empty. So it seems that user id is not present anymore.
However, when I tried to search the same id in other table like _system_users, I did get a hit, however not sure If that is a correct result as the user exists.
yeah. it's unclear to me as to when to link the id to the _users table vs. the _system_users table. But in any case, I think you can assume that it's gone and maybe you can go and republish the dashboard and this should help the problem go away.