Someone had a similar error recently too - Unable to restore/update discourse (PG::InvalidTextRepresentation: ERROR: invalid input syntax for type integer: "[]"), which was due to some stray data in the selectable_avatar site setting field blocking its migration. There is a manual workaround using the rails console, however you do need to be comfortable using the rails console.
If the rails console is out of your comfort zone, @david did mention creating an automatic tweak if there were any more reports. 