Problem with modifying appointment

Hi apetin20,

you can try this options:

Option 1:

To mitigate the issue, you can use the Clear Offline Items for your calendar. This step clears the calendar items in the Outlook store (OST) on your local machine.   

To clear offline Calendar items, follow these steps: 

1. Open the Calendar pane in Outlook, and right-click the Calendar folder.

2. Select Properties.

3. On the General tab, select Clear Offline Items.

4. Select OK.

After you clear offline items, Outlook will re-sync the calendar and repopulate your calendar folder with the calendar items on the server. 

Option 2:  

Make the meeting updates in Outlook Web Access (OWA) or use New Outlook. The conflict does not occur when updating or cancelling the meeting in OWA or New Outlook. 

Outlook shows conflict errors when updating or cancelling meetings - Microsoft Support