can't login after Update (OTRS 5 > 6)

After updating from OTRS 5 to OTRS 6 we can't login.
The login button ist not responding.

here some lines from the Errorlog:
ERROR: OTRS-otrs.Console.pl-Maint::Email::MailQueue-11 Perl: 5.22.1 OS:
linux Time: Mon Jan  8 16:52:02 2018

Message:
CommunicationLog(ID:312,AccountType:-,AccountID:-,Direction:Outgoing,Transport:Email,ObjectLogType:Message,ObjectLogID:608)::Kernel::System::MailQueue
=> Permanent sending problem or we reached the sending attempt limit.
Message will be removed

Traceback (1511):
   Module: Kernel::System::CommunicationLog::_LogError Line: 538
   Module: Kernel::System::CommunicationLog::ObjectLog Line: 306
   Module: Kernel::System::MailQueue::_SendError Line: 853
   Module: Kernel::System::MailQueue::Send Line: 689
   Module: Kernel::System::Console::Command::Maint::Email::MailQueue::Send
Line: 194
   Module: Kernel::System::Console::Command::Maint::Email::MailQueue::Run
Line: 143
   Module: (eval) Line: 460
   Module: Kernel::System::Console::BaseCommand::Execute Line: 454
   Module: (eval) Line: 144
   Module:
Kernel::System::Daemon::DaemonModules::SchedulerTaskWorker::Cron::Run Line:
123
   Module: Kernel::System::Daemon::DaemonModules::SchedulerTaskWorker::Run
Line: 235
   Module: (eval) Line: 316
   Module: main::Start Line: 316
   Module: /opt/otrs//bin/otrs.Daemon.pl Line: 137

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