NSInvalidArgumentException: -[__NSCFNumber count]: unrecognized selector sent to instance 0x8000000000000000 - ios

Question

I am receiving this error from a bug report and have no idea what has caused it.
Here is the complete error message:
Hardware Model: [object Object]
Process: SkippersLog
Identifier: de.sioned.skipperslog
Version: 1.2.1
OS Version: iOS 13.6.1
NSInvalidArgumentException: -[__NSCFNumber count]: unrecognized selector sent to instance 0x8000000000000000
0 CoreFoundation ___exceptionPreprocess
1 libobjc.A.dylib _objc_exception_throw
2 CoreFoundation -[NSObject(NSObject) doesNotRecognizeSelector:]
3 CoreFoundation ____forwarding___
4 CoreFoundation ___forwarding_prep_0___
5 SkippersLog generic specialization <__C.CKRecordID, __C.CKRecord> of Swift.Dictionary._Variant.updateValue(_: __owned B, forKey: A) -> B? (<compiler-generated>)
6 SkippersLog updateValue (<compiler-generated>)
7 SkippersLog closure #1 () -> () in SkippersLog.CloudKitFactory.transferAllTablesToCloud() -> () (CloudKitFactory.swift:203:18)
8 SkippersLog reabstraction thunk helper from #escaping #callee_guaranteed () -> () to #escaping #callee_unowned #convention(block) () -> () (<compiler-generated>)
9 libdispatch.dylib __dispatch_call_block_and_release
10 libdispatch.dylib __dispatch_client_callout
11 libdispatch.dylib __dispatch_root_queue_drain
12 libdispatch.dylib __dispatch_worker_thread2
13 libsystem_pthread.dylib __pthread_wqthread
14 libsystem_pthread.dylib _start_wqthread
The only cause can be the updateRecords.updateValue(ckrecord, forKey: ckrecord.recordID) call in the following function:
func gatherCachedRecords()
{
var ckcaches : [CKCache] = []
*** filling ckcaches from database her ***
updateRecords.removeAll()
for ckcache in ckcaches
{
if let dbrecord = DBRecord.findDBRecord (recid:ckcache.recid)
{
let ckrecord = dbrecord.createCKRecord()
updateRecords.updateValue(ckrecord, forKey: ckrecord.recordID)
}
}
}
updateRecords is a property of the class defined as:
private var updateRecords : [CKRecord.ID: CKRecord] = [:]
gatherCachedRecords is called in global DispatchQueue:
func transferAllTablesToCloud()
{
CKManager.shared.activityIndicator(on: true)
DispatchQueue.global(qos: .utility).async
{
self.dispatchGroup = DispatchGroup()
self.gatherCachedRecords()
*** do some other stuff here ***
}
}
transferAllTablesToCloud is only called once during startup of the app. The only possible problem I could guess is, that updatedRecord is declared on class scope and not locally within the DispatchQueue block, but I have no idea why this could be a problem.
This should only cause trouble if transferAllTablesToCloud would be called more than once and at the same time running concurrently.
The class that hosts all this is a singleton.
I cannot reproduce this in my development environment.
Thanks in advance.