Issue: You Cannot change password on M365 (O365) account.

Though user has login successfully when he tries to change his password, he receives a message that the old password is wrong !!!

Also you receive "Connected data source error: 8334" in Azure AD Connect

Resolution:

The Azure ADC Connect uses a user account named "MSOL_xxxxxxxxxxxx". If you have sub-domains syncing with Azure ADC Connect, this user must be a member of the "Domain Admins" in order password changes to write back (from M365 to your Active Directory).

Things you may also take care:

The user account does not have "Account Never Expires" set.

Set the policy "Minimum password age" to 0, in order to be able to make changes within the day.