It could be any number of things, but typically with the error you're seeing either the data being fed to a column is empty or is of a type that it cannot convert into the type that it wants for the column in question. 

Without more details it's difficult to say how to solve it, but I would start by creating a little test workflow that sends an email to yourself. Specifically I would create (2) Single Line Text Variables to hold the name values we get from User Profile Lookup:

Then just make two Query User Profile Actions, and configure them to get the "Name" property:

(Note: you will need to select the property "Name" first, and then click the Add button to enable it)

Afterwards select a variable from the dropdown, then save.

After you've done that, make the Send Email action to send to yourself and pull the variable data in via reference:

The final workflow would look something like this: 

Once you run the workflow you should be able to at least see that both users are returning *something* back to you. If they aren't, then it would be time to check AD stuff (or to run the jobs for your User Profile Service to pull in fresh data from the AD server). No matter, you should be able to use this workflow to at least check whether or not your changes elsewhere are affecting how sharepoint is able to pull in these user's names. 

I've ran multiple variations of your suggestions with no issues. It puzzles me that the workflow pulls all correct originator information except when updating the Title of the msg to the manager.

The fact that the issue started after same-name-employee2 joined the org makes me think this issue is related to AD stuff. I wanted to check other's perspective  and see if I have missed something before engaging the AD team