Observation Details
Error messages merely duplicate label content ("Vorname" → "Bitte geben Sie Ihren Vornamen ein."). Error messages must inform the user about the error, not about existing information.
Remediation Notes
Instead of repeating label content, explain the actual errors to the user, so the user is helped with solving the issue at hand. This means, using error messages like:
"Dieses Feld darf nicht leer sein."
"Die E-Mail-Adresse enthält kein @ Symbol"
...
Repetition of label content is not necessary, as the errored input label will still be announced to assistive technology. Repeating label content will only duplicate announcement. Navigating to an errored input via assistive technology will announce the label and the description (error message).