My findings are:
1)Job on DBSTU server: Agent history clean up: distribution is failing.
From the Job error I figured out was Table [dbo].[MSmerge_history] on distribution was missing.
This is causing issue? I am not sure...
2)I did a test for small database, it works fine.
The REPL_snapshot, REPL_logreader and REPL_dist are running fine.
The fact that the Agent history cleanup job is failing will not, in itself be the reason you're seeing an error in Replication Monitor. However I'd certainly be very concerned if that table is not there. It should always exist the distribution database, as far as I know, and suggests that you have deeper issues.
Possibly a silly question but it is merge replication you have configured right?
Measure twice; cut once (and have a good saw)
Hey, just a thought.....did you check Books Online yet?