Every few minutes I get a repeat of an alleged synchronisation error

Tim Cummins 20 Reputation points
2026-11-04T15:59:21.1766667+00:00

This message repeats. How do I put an end to this and resolve it?

15:20:14 Synchroniser Version 16.0.20527

15:20:14 Synchronising Mailbox ’Tim Cummins’

15:20:14 Synchronising Hierarchy

15:20:15 Synchronising server changes in folder ’Ajouter_�_votre_agenda.ics’

15:20:15 Downloading from server ’[Moderator note: Personally Identifiable Information removed]’

15:20:15 SyncMonitor triggered for Ajouter_�_votre_agenda.ics (0x80004005)

15:20:15 Error synchronising folder

15:20:15                              [80004005-501-4B9-560]

15:20:15                              The client operation failed.

15:20:15                              Microsoft Exchange Information Store

15:20:15                              For more information on this failure, click the URL below:

15:20:15                              https://www.microsoft.com/support/prodredirect/outlook2000_us.asp?err=80004005-501-4b9-560

Outlook | Windows | Classic Outlook for Windows | For business
Answer accepted by question author

AI answer

2026-11-04T15:59:41.8933333+00:00

This looks like a folder-specific Outlook sync failure. The folder name Ajouter_�_votre_agenda.ics strongly suggests an Internet Calendar subscription (.ics) rather than a normal Exchange mailbox folder, and repeated sync errors can continue until that item or local cache is corrected.

Try these steps in order:

  1. Check whether this is an Internet Calendar subscription.
    • In classic Outlook, look for a subscribed calendar with a name matching Ajouter_�_votre_agenda.ics.
    • If it is an external calendar subscription, remove that subscription and add it again only if needed.
    • If the source ICS feed is malformed, the error can keep returning until the provider fixes the source.
  2. Resync the affected folder.
    • Right-click the affected folder and select Properties.
    • Select Clear Offline Items and then OK.
    • Go to Send/Receive and select Update Folder.
    • This is the supported first step for a specific folder sync problem in Outlook 2010 and later.
  3. Rebuild the Outlook OST cache if the error keeps repeating.
    • Risk: local-only data not present on the server can be lost.
    • If there is any local data not on the server, export it to a .pst first.
    • In Outlook, go to File > Account Settings > Account Settings > Data Files.
    • Note the path of the OST file for the mailbox.
    • Close Outlook.
    • In Windows Explorer, go to that path and delete the .ost file.
    • Restart Outlook and allow the mailbox to fully resynchronize.
    • If Outlook reports the file is in use, open Task Manager and make sure Outlook.exe and Winword.exe are not still running before retrying.
  4. If rebuilding the OST does not fix it, create a new Outlook profile.
    • This is the next documented step when recreating the OST does not resolve sync errors.
  5. If the error is specifically the known 0x80004005 folder sync issue in classic Outlook, check the mailbox server build.
    • Microsoft has already fixed a service-side issue affecting folder sync with 0x80004005.
    • The fix is available in server builds later than 15.21.0155.000.
    • While rollout is ongoing, use Outlook Connection Status and review the Version column to see which server build hosts the mailbox.
    • If the mailbox is still on an older build and this is high impact, contact Microsoft Support.

Because the failing object appears to be an .ics calendar folder, the most likely first action is to remove the Internet Calendar subscription, then test whether the repeated sync message stops. If it does not, proceed with the folder resync and then OST rebuild.

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  1. Jack-Bu 7,220 Reputation points Microsoft External Staff Moderator
    2026-11-04T16:32:56.3733333+00:00

    Hi Tim, 

    Thank you for sharing the details of your situation. 

    The error appears to be linked to a calendar folder or .ics item named “Ajouter_�_votre_agenda.ics”. Classic Outlook is repeatedly attempting to synchronize this item with the Exchange server but cannot complete the operation, which is why the error reappears every few minutes. 

    Here are some steps you can try: 

    1/ Check the calendar in Outlook on the web 

    1. Sign in at https://outlook.office.com. 
    2. Open Calendar and look for a calendar or subscription with a name similar to “Ajouter à votre agenda.” 
    3. If you find it and no longer need it, right-click it and select Remove, Delete, or Unsubscribe, depending on the available option. 
    4. Close and reopen Classic Outlook, then check whether the synchronization error stops. 

    2/ Test Classic Outlook in Safe Mode 

    1. Close Classic Outlook. 
    2. Press Windows key + R. 
    3. Enter outlook.exe /safe, then select OK. 
    4. Check whether the synchronization error continues. 

    If the issue does not occur in Safe Mode, an Outlook add-in may be interfering with synchronization. To disable the add-ins: 

    1. Open Classic Outlook normally. 
    2. Go to File > Options > Add-ins. 
    3. At the bottom, next to Manage, select COM Add-ins, then select Go. 
    4. Clear the checkboxes for the enabled add-ins, then select OK. 
    5. Restart Outlook and check whether the issue is resolved. 

    If the error stops, re-enable the add-ins one at a time and restart Outlook after each one. This should help identify which add-in is causing the issue. 

    3/ Create a new Outlook profile 

    If the issue also occurs in Safe Mode, or disabling the add-ins does not help, create a new Outlook profile: 

    1. Close Classic Outlook. 
    2. Open Control Panel > Mail (Microsoft Outlook). 
    3. Select Show Profiles > Add. 
    4. Enter a name for the new profile and follow the prompts to add your email account. 
    5. Select Prompt for a profile to be used, then select Apply > OK. 
    6. Open Classic Outlook, choose the new profile, and allow the mailbox to synchronize. 
    7. Check whether the synchronization error occurs in the new profile. 

    You may wish to select Prompt for a profile to be used so that you can switch between the original and new profiles while testing. 

    If the issue persists in the new profile, please let me know whether your emails and other folders are synchronizing normally and whether the affected calendar is visible in Outlook on the web. 

    I hope this information is helpful. Should you have any further questions or need additional assistance, please feel free to share them in the comment below. I'm very happy to help.  

    If the answer is helpful, please click 'Yes' and kindly upvote it. 

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

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