All the servants' "work" idle markers in SolitudeBluePalace cell are set to "Owned by SolitudeBluePalaceServants faction" by Skyrim.esm. The problem is that the faction has no members assigned to it, so the servants (Erdi, Odar, Una) do nothing during their working hours, they just stand still in their sandbox area.
I'd like to suggest 2 options to fix the issue if you don't mind:
Option 1: Assign Erdi, Una and Odar to SolitudeBluePalaceServants faction. Cons: it will take some time for mods that override these NPCs (for example, the fairly popular "Pandorable NPCs" from my mod list) to catch up with the change.
Option 2: Change the ownership of the idle markers. The servants seem to work at different parts of the cell, so, for example, assigning all the kitchen markers to Odar instead of SolitudeBluePalaceServants could do the trick.