​​​​Microsoft has shared a workaround for a known issue affecting Microsoft 365 customers and causing classic Outlook to crash after opening or when starting up in Safe mode.

Impacted users can confirm if they’re affected by this specific issue by looking for Event 1000 or Event 1001 crashes in the Windows Event Viewer Application Log with 0xc0000005 exception codes linked to the faulting module ucrtbase.dll.

Based on user reports, the crashes impact only Office 365 accounts, and they cannot be solved by reverting to the previous build, creating a new Outlook profile, or even after a Windows clean install.

After investigating this, Redmond says the issue is linked to Microsoft 365 email accounts and corrupted mailbox client and server rules.

“After updating to Version 2407 Build 17830.20138 or higher you find that Outlook may unexpectedly close at start up. Outlook will also close if you test in Safe Mode,” the company said in a support document published on Friday.

“This issue has been found to occur if the server-based rules for the M365 email account are corrupted or that Outlook cannot process.”

Until an official fix is released for this issue, Microsoft provides a temporary workaround requiring classic Outlook users to delete all email rules linked to their accounts manually.

To do that, you will have to:

1. To remove the client and server rules, run the “clean rules” command line by closing Outlook, right-clicking the Windows Start button, selecting Run, and running an ‘Outlook.exe /cleanrules’ command.
2. Hold the Shift key when starting Outlook to create a new profile, but only if necessary. On the Profile Picker dialog, click Options, check “Prompt a profile to be used,” then click “New” and proceed through the prompts (official guidance is available here).
3. If you still have the issue, go to Outlook Web Access and manually remove all the email rules.

​This week, Redmond also shared a workaround for an issue blocking Microsoft 365 customers from signing in or adding Gmail accounts in classic Outlook.

Three days later, the company also provided a temporary fix for an issue causing Microsoft 365 apps like Outlook, Word, and OneNote to crash while typing or spell-checking a text.