The error message "Inhalt syntaktisch ungültig." is shown, if your password has special characters, that are not supported from the FinTS interface (e.g. ~ or €). 

Please change your password on the Onlinebanking website of your bank, to be able to add the credentials in Outbank.
These special characters should work:
!§$%&/()=?@_-ß{[]}\<>,;.:#+*)