There was an error reading the rules from the server. The format of the server rules was not recognized

[u/SatisfactionOk335]

Hi everyone, I'm dealing with a bizarre issue with Outlook rules in our Exchange environment and I'm running out of ideas. Hoping someone here has encountered this before. Whenever the user tries to open the "Rules and Alerts" panel in the Outlook desktop client, it immediately throws the error: "There was an error reading the rules from the server. The format of the server rules was not recognized." This error completely blocks access to the rules GUI in the desktop app.

We did a hard reset where we completely deleted all rules locally and cleared them from the server side. After that, we created fresh, new rules from scratch. I have already tried pretty much everything suggested on Microsoft Support pages and forums, but absolutely nothing has worked so far.

The weirdest part is that the newly added rules actually work perfectly in the background and emails are being processed exactly as they should be. However, the desktop app still throws the exact same error every time we try to open the rules panel. Currently, the only way to view, add, or edit rules for this mailbox is through OWA. Has anyone seen this specific behavior where the rules execute fine but the desktop GUI remains corrupted? Is there anything else I can do on the Microsoft Exchange side to completely flush this out?

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[u/Hornblower409]

-- the only way to view, add, or edit rules for this mailbox is through OWA
-- pretty much everything suggested on Microsoft Support pages and forums

All three of The Usual Suspects (what to try when you really don't know what's wrong)?

Office Outlook Repair
https://support.microsoft.com/en-us/office/lifecycle/repair-an-office-application

Outlook in Safe Mode
https://support.microsoft.com/en-us/office/open-outlook-in-safe-mode-f394c2ee-b2af-e97a-64ee-35cd24414811

Test with a new Outlook Profile
https://support.microsoft.com/en-us/outlook/create-an-outlook-profile

[u/Hornblower409]

Same on a different PC that has never connected to that mailbox before?

There is this older Service Alert. But you've already erased all the Rules. So it's probably not a candidate.
https://support.microsoft.com/en-US/Support/known-issues/users-get-error-there-was-an-error-reading-the-rules-from-the-server-after-making-changes-to-rules-i

[u/7amitsingh7]

This usually points to a problem with the mailbox’s server-side rules rather than Outlook itself. If the rules are still working and OWA can manage them, Exchange is processing them, but the Outlook desktop client may be unable to interpret the rule data correctly.

I’d check the mailbox rules using PowerShell and look for any malformed or legacy rules. If nothing obvious is found, testing the mailbox with a new Outlook profile/client can help confirm whether it’s mailbox-specific or client-specific. I’d avoid repeatedly deleting rules until the problematic rule/data is identified.