eM Client 9.2.2307: SMTP error

Dear reader,

Occasionally I’m receiving this error when sending email using SMTP. Is this a bug in eM Client? Because when I wait some time (mostly minutes) and retry (without having changed or touched any SMTP setting) the email gets send.

The SMTP server is a private one from our ISP. So not Office365 or something like that.

09:50:04 Online toestand: gewijzigd in online vanwege NetworkAvailability 
09:50:04 ...@... IMAP Mappenlijst aan het synchroniseren
09:50:04 ...@... SMTP Aan het verbinden: Naar ...@...
09:50:04 ...@... IMAP Map '/Postvak IN aan het synchroniseren
09:50:05 ...@... [SMTP] MailClient.Accounts.LoginException: De authenticatie is mislukt vanwege de volgende reden: 
            "The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters.
            at MailClient.Protocols.Smtp.SmtpSendCommand.Authenticate(WorkerStatus status)
            at MailClient.Protocols.Smtp.SmtpSendCommand.Connect(WorkerStatus status)
            at MailClient.Protocols.Smtp.SmtpSendCommand.Execute(WorkerStatus status)
            at MailClient.Commands.Command.Process(WorkerStatus status)
           EnqueuedStackTrace =
                at MailClient.Protocols.Smtp.SmtpAccount.<>c__DisplayClass5_0.<SendAsync>b__0()
                at MailClient.Protocols.Common.AccountBase.RunIfOnline(Action actionIfOnline, Action actionIfOffline)
                at MailClient.Protocols.Smtp.SmtpAccount.SendAsync(IEnumerable`1 items, Action`1 sentItem, Action`1 completed, CancellationToken cancellationToken)
                at MailClient.Accounts.BindingAccountBase.SendInternal()
                at MailClient.Accounts.BindingAccountBase.Send()
                at MailClient.Accounts.AccountManager.SendAllInternal(Boolean checkIncludeInGlobalOperations)
                at MailClient.Accounts.AccountManager.SendAndReceiveAll(Boolean checkIncludeInGlobalOperations)
                at MailClient.Program.<>c.<AfterStartup>b__279_0(Task _)
                at System.Threading.Tasks.ContinuationTaskFromTask.InnerInvoke()
                at System.Threading.Tasks.Task.<>c.<.cctor>b__272_0(Object obj)
                at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(Thread threadPoolThread, ExecutionContext executionContext, ContextCallback callback, Object state)
                at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread)
                at System.Threading.Tasks.Task.ExecuteEntryUnsafe(Thread threadPoolThread)
                at System.Threading.Tasks.Task.ExecuteFromThreadPool(Thread threadPoolThread)
                at System.Threading.ThreadPoolWorkQueue.Dispatch()
                at System.Threading.PortableThreadPool.WorkerThread.WorkerThreadStart()
                at System.Threading.Thread.StartCallback()
           GUIStatus_Exception_Reported = True

Kind regards,
Mirco Babin