If I sign a jar with the ANT task fx:signjar it doesn't the jar, if it doesn't contain a META-INF/MANIFEST.MF file.

At least this is my suspicion. The jar in question is javax-inject-1.jar:

<dependency>
  <groupId>javax.inject</groupId>
  <artifactId>javax.inject</artifactId>
  <version>1</version>
</dependency>

The normal sign task works.

The result is, that I can't launch the JNLP file.

It exits with this error:

network: Keine Zertifikatsinformationen für nicht signierte JAR-Datei: file:/.../javax.inject-1.jar
cache: Done readSigners(file:/.../javax.inject-1.jar)
security: Found unsigned entry: javax/inject/Inject.class
preloader: Delivering: ErrorEvent[url=file:/.../App.jnlp label=Unsignierter Eintrag gefunden in Ressource: file:/.../javax.inject-1.jar cause=Unsignierter Eintrag gefunden in Ressource: file:/.../javax.inject-1.jar
basic: Ausnahme: Unsignierter Eintrag gefunden in Ressource: file:/.../javax.inject-1.jar.
ExitException[ 3]com.sun.deploy.net.JARSigningException: Unsignierter Eintrag gefunden in Ressource: file:/.../javax.inject-1.jar
at sun.plugin2.applet.JNLP2Manager.prepareLaunchFile(Unknown Source)
at sun.plugin2.applet.JNLP2Manager.loadJarFiles(Unknown Source)
at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)

(it is german, it means "unsigned entry found in resource")