When using eM Client, especially when it's set up to use both IMAP for email and WebDAV for calendars, it's possible notifications from attendees who have ACCEPTED your meeting/appointment invitations will display something like this (note the highlighted section):
The meeting was accepted, but the reply notification states that the scheduled items "were not found." This can be somewhat confusing as you know you made the appointment and sent out the invitations.
Generally, this occurs when the account was configured in eM Client separately: it was first set up for mail as an IMAP account, then the same account was set up for calendaring using WebDAV. When this happens, you see some unique behaviors like the one noted above.
To fix this issue, the accounts will need to be removed from eM Client, then re-added using the auto-configuration available when adding a new account. Ideally, this will set up the account in eM Client using EWS, if it's available. If not, the account will be added so BOTH IMAP and WebDAV are used. The benefit is that the account is set up once, but utilizes both IMAP and WebDAV versus having the account set up multiple times.
Regardless, setting up the account ONCE will fix any oddities seen.