New bug in classic Outlook can only be fixed via Microsoft support

Microsoft is investigating a known issue causing the classic Outlook client on Windows to crash at launch for some Microsoft 365 customers; affected users are advised to open an Exchange Online support case so the service team can request a mitigation. Microsoft recommends capturing a Fiddler trace for the specific "LID:49586 - Authentication concurrency limit is reached." error and suggests using the new Outlook for Windows or Outlook Web Access (OWA) as temporary workarounds while troubleshooting steps and fixes are pursued.