While doing some research on the NPC Erikur, I found something strange in the changelog.
Erikur's house in Solitude should not have been made a valid target for Thieves Guild jobs. It's marked with a "protected" shadowmark and the theft markers inside are not setup up correctly. (Bug #14905, Bug #14904, Bug #14903, Bug #14638, Bug #14158)
Erikur's house in Solitude was configured to be a Thieves Guild radiant location but lacked the appropriate chest and markers to make it work properly. (Bug #13155)
Seems like USSEP both makes the location work with the Thieves Guild radiant quests while also stopping it from being picked. My guess is that either the edits to the chest and marker should be removed, or it should be made available for the quest after all (though that would ignore the shadowmark).