Same version of DB for dump & restore? If not, was the dump done via the pg_dump from the newer version. If not, please do that.
I'm using the same version. I make this test on the same machine.
Moreover I try to remove this line using pgadmin and I get the same error (this field is part of primary key).
To remove this line I need to not use primary key but give a where condition that return only this tuple.
Again, is very strange that this data is in the database .... moreover this data came from a import procedure and this data is not present in the source import file.
Really, I think that I get some kind of data corruption