There is a dagger display case activator added by USSEP (Bug #13152) which respawns and can be problematic if it has any other dagger placed in it besides the original Dwarven Dagger. Once the Dwarven Dagger is replaced with another dagger, when the cell respawns both the newly placed dagger and a respawned Dwarven Dagger appear in it. Additionally, taking the new dagger from the activator results in the activator becoming inaccessible. This problem exists even if the activator doesn't have a respawning Dwarven Dagger in it. Leaving the cell and returning to it fixes it, but needless to say activators have issues if they respawn.
It probably would have been preferable to not add an activator in this particular display case, keeping it consistent with the display cases in Jorrvaskr Harbinger's study room (although those are a little different, containing leveled weapons and always accessible) and the Hearthfire homes (those don't have any weapons in them to suggest that this is what they were intended for). If the activator is to stay, I have found that flagging the activator (the Dwarven Dagger might need to be flagged as well) as non-respawning fixes the issue.
Issue Data
Issue #36766: Dagger display case activator in the Archmages Quarters