I forgot to mention what I actually did.

Since I don't have any certificates, I tried removing the JNLP.
I removed the JNLP folder in the JAR and I removed every references to the JNLP file from the MANIFEST file inside the META-INF folder.

After this, I repacked the JAR file.
I removed the references to the JNLP from the HTML file and tried to load it.

After the app has loaded, I get the error previously mentionned.