NSKeyedUnarchiver.unarchivedObject(ofClass... returns nil although its deprecated predecessor function worked

Question

I have old code that uses NSKeyedUnarchiver.unarchiveTopLevelObjectWithData to unarchive data.

let unarchivedData = try? NSKeyedUnarchiver.unarchiveTopLevelObjectWithData(archivedData)

unarchivedData then holds a NSDictionary that contains Strings, Ints, Dictionaries and NSData stuff.

As this function is deprecated I now use this function

let unarchivedData = try? NSKeyedUnarchiver.unarchivedObject(ofClass: NSDictionary.self, from: archivedData)

but unarchivedData now contains nil.

I have no idea why this new function does not work any more.
What could be the reason for this? Also using try/catch shows no hint in the error variable.