Email provider GMX allows its users to enable more secure authentication for their accounts, the two-factor authentication - https://www.gmx.com/security/2fa
When you enable this additional security layer, you need an app password for other apps to login into your GMX account for you.
Most email service providers will use the same app password for one app, but GMX needs this password per each protocol/service you plan to use in a 3rd party application.
That means that for eM Client you need three app passwords total - one to receive email via IMAP service, one to send via SMTP, and one to synchronize your contacts and calendars via CalDAV/CardDAV service.
Try to use the same password on the whole account will result in rejected logins.