Microsoft confirms Classic Outlook crashes on startup, offers a workaround

Just a couple of days ago, Microsoft confirmed that certain Office apps could crash when doing mundane tasks, such as typing or spell-checking. According to Microsoft, the bug affects Office version 2407, namely OneNote, Word, and Outlook. Sadly, that is not the only crash-inducing bug in the Classic Outlook app.

In another support document, Microsoft revealed that Classic Outlook in version 2407 (build number 17840.20138) may "unexpectedly close at start up." In other words, crash upon launch. The same may happen when attempting to launch Outlook in Safe mode.

If your Classic Outlook crashes on launch, you can check for the following details in Windows Event Viewer:

You can confirm if this is the issue by looking at the Windows Event Viewer Application Log for crash Event 1000 or Event 1001, and the following event details:  

Fault Bucket 1620625936954482602, Type 4
Faulting application name: OUTLOOK.EXE, version: 16.0.17830.20138, time stamp: 0x66aaad8c
Faulting module name: ucrtbase.dll, version: 10.0.22621.3593, time stamp: 0x10c46e71
Exception code: 0xc0000005
Fault offset: 0x000000000005137c
Faulting process id: 0x0x5A00
Faulting application start time: 0x0x1DAE741D3515015
Faulting application path: C:\Program Files\Microsoft Office\root\Office16\OUTLOOK.EXE

Microsoft says that the root cause of the problem is corrupted server-based rules for Microsoft 365 mail accounts, which Outlook cannot properly process (ucrtbase.dll). Fortunately, there is a workaround:

To work around the issue, do the following:

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

You can find more information about the issue in the official support document. Also, check out a forum thread using this link.