So I resolved the problem (not sure the root issue) by following these steps :
http://kb.palm.com/wps/portal/kb/com.../19887_en.html
Removing duplicates from sync accounts
To remove duplicate calendar data, delete the account you are synchronizing with from your phone, clean up any duplicate information in your sync account online, and then add the account to your phone again.
1. Open Backup icon.
2. Turn off automatic backup, and delete backed-up information:
3. Tap On to switch Backup from On to Off.
4. Tap Turn Off And Erase Data. This step deletes all your information on the server but does not affect the information on your phone.
5. Open Calendar icon.
6. Open the Application menu, and tap Preferences & Accounts.
7. Under Accounts, tap the account name.
8. Tap Remove Account, and then tap Remove Calendar Account.
Note Deleting a sync account stops synchronization of calendar events only—if you also sync contacts or email messages with the deleted online account, those items continue to synchronize.
9. Next, log in to your sync account, such as Google or EAS, and remove any duplicate data.
10. When the duplicates are removed from the sync account, add the account to Calendar again.
11. Screenshot First Use Open Calendar .
12. Tap the account type, enter the account information, and tap Sign In.
13. Tap Done. This opens your Palm Profile account.
14. Open Backup icon.
15. Tap Off to switch backup from Off to On.