Outlook error: format of the server rules was not recognized

Problem

Out of the blue, my Outlook Windows application threw this error dialog when I opened the application:

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

I tried closing and reopening the application and every time the error repeated. I started to check my rules on each restart and noticed that my rule list was getting corrupted from the end. The names of the rules at the end were being replaced with the string (client-only).

I checked Outlook on the web, and the rules there too were starting to look corrupted. Essentially, for some reason the rules on Microsoft Exchange had gotten corrupted and it was slowly destroying my rule list.

Solution

There are many solutions online and this is the only one that worked for me:

• First and foremost, you want to save your rules! Export them to a .rwz file from the Outlook Windows application.
• Delete all the rules. Restart Outlook.
• If you still see the error dialog and/or you still a single (client-only) rule, delete it.
• Create a new rule and save it. Restart Outlook.
• On restart, Outlook will ask this question:

The rules on this computer do not match the rules on Microsoft Exchange. Only one set of rules can be kept. You will usually want to keep the rules on the server. Which rules do you want to keep?

• Choose the client option. Restart Outlook.
• Import the old saved rules from the .rwz file. I found that a couple of the rules at the end of the list were still titled (client-only). I just deleted them, hoping for the best.
• Restart Outlook and everything should be fine now.

Reference

• https://answers.microsoft.com/en-us/outlook_com/forum/all/there-was-an-error-reading-the-rules-from-the/300fdbb3-f67a-4731-b40a-e519cc808953