If your outlook.com account was added to eM Client by Automatic Setup, it unfortunately uses the AirSync protocol to sync the calendar and contacts. I have never found that option very reliable.
What you can do is remove the account from eM Client, then add it again as an Exchange account. Instead of entering your email address in the Automatic Setup, choose Mail > Exchange. This uses EWS, which is very reliable. Exchange accounts have many other benefits over IMAP/AirSync.
have now installed exchange. Unfortunately, I continuously receive a 503 error message. After two hours, my agenda is not yet visible. Am I doing something wrong?