Ideas
POSSIBLE FIX! I was having this same issue today as well. Mine was also an updated version of a previously uploaded lot. I uploaded a lot, realized I forgot the mailbox (as you do), so I deleted the old version off the gallery, saved a new version to my library, overwriting the old version, and then was getting the error when I tried to upload the new, overwritten version.
But when I saved a brand new version of the lot to my library instead of overwriting the previous version, I was able to upload it just fine. Hope this helps someone!
roguelynx96 ryazan_atra Thank-you both of you for sharing your work around method! Ran into this error for the first time today (18 Dec, so patch 1.120) with a household I wanted to share. Similiar story, I wanted to fix a typo, so I deleted it off the gallery and then ran into this error trying to re-upload. Saving a fresh copy of the household into my Library and then uploading that one to the Gallery worked.