I think the code below is what's failing. I got this by running sp_helptext sp_changedistributiondb & also looking at what sproc's call sp_verify_job_identifiers.
It looks like changedistributiondb is having trouble updating the distribution cleanup agent job when it runs sp_update_jobstep :
SELECT @command = 'EXEC dbo.sp_MSdistribution_cleanup @min_distretention = ' +
CONVERT(nvarchar(12), @new_min_distretention) + ', @max_distretention = ' +
EXEC @retcode = msdb.dbo.sp_update_jobstep @job_name = @agentname, @step_id = 1,
@command = @command
maybe this job has been deleted or there is something else strange going on, either way, you should be able to modify the job step manually.
EDIT: The cleanup agent runs on the server with the distribution database. It removes replication history and runs every 10 minutes by default. The job should look like this:
name: Agent history clean up: distribution
category: REPL-History Cleanup
steps: (1)Run Agent
command:EXEC dbo.sp_MShistory_cleanup @history_retention = 48