First of all NSKeyedUnarchiver should be allocated and initialized like this (objective-c): NSError* err; NSKeyedUnarchiver* unarchiver = [[NSKeyedUnarchiver alloc] initForReadingFromData:someData error:&err]; It looks like you use NSSecure coding. Therefore you have to specify classes on decoding objects. In your case it looks something like this like this NSDictionary* someDict = [unarchiver decodeObjectOfClass:[NSDictionary class] forKey:NSKeyedArchiveRootObjectKey]; But the NSDictionary contained additional strings. That was the cause of the log message. By adding NSString to the decoded class list the error is gone. NSDictionary* someDict = [unarchiver decodeObjectOfClasses:[[NSSet alloc] initWithArray:@[[NSDictionary class],[NSString class]]] forKey:NSKeyedArchiveRootObjectKey] This is a copy from my answer here: https://stackoverflow.com/questions/70050018/error-general-nskeyedunarchiver-validateallowedclassforkey-allowed-u Edit: It's well known that there was an old version of firebase/crashlytics (around 2016) that had this bug, so updating should just be it.

@JMDelh's recommendation worked for me. Tested on M1 Pro / Monterey, Xcode 13.1