I Believe that unknown is from some Migration that had a UUID that does not exist on yjis Install, normally the first user is UUID 501, 502 for the next & so on, say this unknown user was 503 on the other Disk/Mac, then this Mac has no idea who 503 is or what to do with it.
Just thought of this, but you my be able to get rid of it by creating a new account or 2, & getting rid of them after removing unknown.
Still, that isn't the problem here unless maybe they're the only owner or user with R&W rights.
What is, where is this folder & what rights does who have?
Mihgt try tis...
Open System Preferences>Accounts, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.
That'll add a 502 UUID account, log out & back into the 1st accunt, can you minus the unknown now?