Like the new Outlook, the Classic Outlook is also affected by a strange bug, but there is a workaround

After a bug affected Outlook, Word, and OneNote, Microsoft revealed that the issue also impacted Classic Outlook. Certain Office apps, including OneNote, Word, and Outlook, crashed last week due to a bug in Office version 2407. However, the bug caused more damage to Classic Outlook, which has been reported to crash upon launch or even when trying to start it in Safe mode.

If you’re tech-savvy, you might head straight to the Windows Event Viewer to investigate the crash logs. You’d be looking for Event 1000 or Event 1001, with a keen eye on details like the faulting application name (OUTLOOK.EXE) and the faulting module name (ucrtbase.dll).

Microsoft says corrupted server-based rules for Microsoft 365 mail accounts, which Outlook can’t seem to process correctly, might cause the issue.

However, the Redmond-based tech giant has offered a workaround that might save your day. The first step is to run the Clean rules command line to clean the client and server rules slate.

If that doesn’t work, you can create a new Outlook Profile. If you’re still stuck, you can visit Outlook Web Access to manually remove any pesky rules.

To work around the issue, do the following: 

1. Run Clean rules command line to remove the client and server rules.
   1. Close Outlook.
   2. Right-click the Windows Start button and select Run.
   3. In the Run box paste: Outlook.exe /cleanrules.
   4. Select OK.
2. Create a new Outlook Profile, if necessary. For more information, please go to Create an Outlook profile.
3. If you still have the issue, please go to Outlook Web Access and manually remove any rules.

You can also see the original forum post leading to the issue with Classic Outlook.