my calendar events are not syncing properly with Exchange Online

Hi Kristina Dennis

Thank you for reaching out to the Microsoft Q&A forum and for providing such a clear and detailed description of the calendar synchronization issue you're facing.

Based on your description, it appears that Outlook is receiving information from the Exchange Online server but may be encountering issues when attempting to send its own changes. The issue could be related to the local synchronization process, a specific error, a misconfigured setting, or a conflict within your Outlook environment.

Here are some recommendations you can try: 

Check the "Sync Issues" Folder: Outlook logs all synchronization problems in a special folder. Finding an error message here is the fastest way to identify the exact cause. 

• In Outlook, switch to the Folder List view.  
• Expand the folder list for your account and look for a folder named Sync Issues. These messages often contain specific codes or descriptions that can pinpoint the problem. 

When Outlook syncs with your Exchange account, it doesn't just sync your calendar. It attempts to synchronize your entire mailbox, email, contacts, tasks, and calendar - all as part of the same process. 

The Sync Issues folder serves as the master log where Outlook records any problems encountered during the synchronization process. That's why an error message about "Public Folders" can be the direct cause of a "Calendar" problem. By checking that folder, you can see exactly where the synchronization process is encountering issues

Test Outlook in Safe Mode to Isolate Add-ins: Add-ins are a common cause of synchronization interference. You can run Outlook in Safe Mode temporarily disables them. 

• Ensure Outlook is completely closed > Press the Windows Key + R to open the Run dialog box. 
• Type outlook.exe /safe and press Enter, then Outlook will open with "Safe Mode" in the title bar > Create a test calendar event and check if it syncs to Outlook on the Web.  
  • If it syncs correctly: The problem is an add-in. You can manage them via File > Options > Add-ins > COM Add-ins. Disable them all and re-enable one by one to find the culprit. 
  • If it still doesn't sync: Please try to the next steps. 

Step 1: Toggle the "Shared Calendar Improvements" Setting: Toggling it can sometimes reset a faulty sync state. 

• In Outlook, go to File > Account Settings > Account Settings > Double-click your email account, click More Settings..., and go to the Advanced tab. 
• Find the checkbox for Turn on shared calendar improvements. If it's checked > uncheck it. If it's unchecked, check it > Click Apply, then OK. Restart Outlook and test the sync again. 

Step 2: Perform an "Online Mode" Test:  

• After double-clicking your email account > Uncheck the box for Use Cached Exchange Mode... and restart Outlook when prompted. 

Once restarted, create a test event. It should appear on the web almost instantly. 

These actions confirm your connection is fine. Afterward, you can go back and re-enable Cached Exchange Mode and try again. 

Since you noted this began after recent updates, if these steps above do not work, you can consider revert to a previous, stable version.

Please start with Sync Issues, the information from there can be very insightful. Please let me know what you find, or if any of the other steps resolve the problem. 

To better understand and assist you more effectively, could you please let me know whether this issue is impacting multiple users or just a specific group? Additionally, is the shared calendar being accessed by external users outside your organization?

The reason I'm asking is that, according to this Microsoft article: Calendar sharing in Microsoft 365 - Microsoft Support - when a calendar is shared between Microsoft 365 users in different tenants, and external sharing is enabled, changes may sync periodically. In such cases, recipients won’t see updates immediately, but the shared copy will be refreshed every few hours. 

I sincerely hope this information helps clarify your situation. Please don’t hesitate to reply if you have any updates or further questions.

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment". 

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.