NSKeyedArchiving issue

I have a large code that I try to update to change deprecated APIs.

In the former version, I used forWritingWith and forReadingWith

    let data = NSMutableData()
    let archiver = NSKeyedArchiver(forWritingWith: data)
    archiver.encode(myObject, forKey: theKey)

if let data = NSMutableData(contentsOf: anURL) {
  let unarchiver = NSKeyedUnarchiver(forReadingWith: data as Data)
        let myObject = unarchiver.decodeObject(forKey: theKey) as! TheObjectType // <<-- returns the object

That I changed to

    let data = NSMutableData()
    let archiver = NSKeyedArchiver(requiringSecureCoding: true)
    archiver.encode(myObject, forKey: theKey)

 if let data = NSMutableData(contentsOf: anURL) {
        do {
            let unarchiver = try NSKeyedUnarchiver(forReadingFrom: data as Data)
           let myObject = unarchiver.decodeObject(forKey: theKey) as? TheObjectType // <<-- This returns nil

This builds correctly.

But on execution, unarchiver.decodeObject now returns nil.

I have searched extensively to find the cause to no avail.

I may probably change the design to avoid NSKeyedArchiver, but that would be a huge refactoring.

I probably miss something obvious.

Could someone hint at the possible cause ?