Malloc double free for ptr - error

Using Swift
3

I do not use Unmanaged or UnsafePointer directly.
I have reduced the code to post here with error; - needed struct PearsonHash is on GitHub:

I get the malloc error after a add in func case4 the lines for OperationQueue as same as in func case5.

import Foundation

@available(macOS 10.15.4, *)
@main
public struct HashSequence {
    
    public static func main () {
        let instance = HashSequence()
        instance.computeTest()
    }
    
    private func computeTest () {
        let _ = collision(length: 4, hash: 0) // Laufzeit rund 5:03 Minuten
    }
    
    public func collision (length : Int, hash expected : UInt8) -> [[UInt8]] {
        var result : [[UInt8]] = []
        switch length {
        case 4:
            result = case4(hash: expected)
        default : print ("NOT YET IMPLEMENTED")
        }
        return result
    }
    
    private func case4 (hash expected : UInt8) -> [[UInt8]] {
        var result : [[UInt8]] = []

        let operations2 = OperationQueue()

        for b0 in 0...255 as ClosedRange<UInt8>{
            let parallel = BlockOperation(block: {
                for b1 in 0...255 as ClosedRange<UInt8>{
                    for b2 in 0...255 as ClosedRange<UInt8>{
                        for b3 in 0...255 as ClosedRange<UInt8>{
                            let data = [b0,b1,b2,b3]
                            if let _ = PearsonHash.isHash(input: data, expectedHash: expected) {
                                result.append(data)
                            }
                        }
                    }
                }
            })
            operations2.addOperation(parallel)
        }
        operations2.waitUntilAllOperationsAreFinished()
        return result
    }
    
    private func case5 (hash expected : UInt8) -> [[UInt8]]{
        var result : [[UInt8]] = []

        let operations = OperationQueue()

        for b0 in 0...255 as ClosedRange<UInt8>{
            let parallel = BlockOperation(block: {
                for b1 in 0...255 as ClosedRange<UInt8>{
                    for b2 in 0...255 as ClosedRange<UInt8>{
                        for b3 in 0...255 as ClosedRange<UInt8>{
                            for b4 in 0...255 as ClosedRange<UInt8>{
                                let data = [b0,b1,b2,b3,b4]
                                if let _ = PearsonHash.isHash(input: data, expectedHash: expected) {
                                    //result.append(data)
                                }
                            }
                        }
                    }
                }
            })
            operations.addOperation(parallel)
        }
        operations.waitUntilAllOperationsAreFinished()
        
        return result
    }
}