If you select the system member store changedBy on an entity
It will save this data (in an association) for you each time a change is made and committed on that entity.
I've seen this error as well in one of the apps I work on (points to a different specialisation of System.User), but haven't managed to reproduce a scenario where this occurs. My assumption is that you had (even temporary) the MendixSSO module with the entity MendixSSOUser in your project, a user of that specialisation changed the object StaffingBlock and the association was automatically saved. The module was removed / the user was removed and Mendix did not properly update this join table (where the foreign key relations between StaffingBlock and MendixSSOUser were stored).
It's not really an issue (the app won't do anything different), but it is annoying to see it occur often in the logs.
What you can do is retrieve those objects from the database and make a new change/commit so that the changedBy association gets updated to an existing user.