datanucleus neo4j - Unable to obtain lock on store lock file - jpa

Question

I am developing an WebApp that uses datanucleus as JPA - Provider and Neo4j as No-SQL database.
My dependencies are
<dependency>
<groupId>org.datanucleus</groupId>
<artifactId>datanucleus-core</artifactId>
<version>[5.0.0-m1, 5.9)</version>
</dependency>
<dependency>
<groupId>org.datanucleus</groupId>
<artifactId>datanucleus-api-jpa</artifactId>
<version>[5.0.0-m1, 5.9)</version>
</dependency>
<dependency>
<groupId>org.datanucleus</groupId>
<artifactId>javax.persistence</artifactId>
<version>2.1.0</version>
</dependency>
<dependency>
<groupId>log4j</groupId>
<artifactId>log4j</artifactId>
<version>[1.2, 1.3)</version>
</dependency>
<dependency>
<groupId>org.datanucleus</groupId>
<artifactId>datanucleus-neo4j</artifactId>
<version>[5.0.0-m1, 5.9)</version>
</dependency>
<dependency>
<groupId>org.neo4j</groupId>
<artifactId>neo4j</artifactId>
<version>2.3.0</version>
</dependency>
My persistence.xml looks like
<persistence-unit name="neo4j">
<provider>org.datanucleus.api.jpa.PersistenceProviderImpl</provider>
<class>entities.Person</class>
<class>entities.Car</class>
<exclude-unlisted-classes/>
<properties>
<property name="javax.persistence.jdbc.url" value="neo4j:C:\Users\phe\Documents\Neo4j\sampledatanucleus"/>
<!-- <property name="datanucleus.storeManagerType" value="neo4j"/> -->
<property name="datanucleus.storeManagerType" value="neo4j"/>
</properties>
</persistence-unit>
I am persisting the entities like this
Person person = new Person(personName, personAge);
Car car = new Car(carName);
car.setOwner(person);
person.getCars().add(car);
em.persist(person);
When persisting the entities I get an exception:
Caused by: org.neo4j.kernel.StoreLockException: Unable to obtain lock on store lock file: C:\Users\phe\Documents\Neo4j\sampledatanucleus\store_lock. Please ensure no other process is using this database, and that the directory is writable (required even for read-only access)
at org.neo4j.kernel.StoreLocker.storeLockException(StoreLocker.java:93)
at org.neo4j.kernel.StoreLocker.checkLock(StoreLocker.java:85)
at org.neo4j.kernel.StoreLockerLifecycleAdapter.start(StoreLockerLifecycleAdapter.java:44)
at org.neo4j.kernel.lifecycle.LifeSupport$LifecycleInstance.start(LifeSupport.java:528)
... 153 more
This makes me crazy. The directory I defined in the perstistence.xml is empty when I start the applictaion. Therefore it is impossible that it is locked by another process. DataNucleus then creates the database. I can access it with Neo4j Browser. But Persisting does not work.
Any ideas?
EDIT:
This is the class I use for querying
#Stateless
public class Manager {
#PersistenceContext(unitName = "neo4j")
private EntityManager em;
public List<Person> queryCache() {
String statement = "Select o from Person o";
Query query = em.createQuery(statement, Person.class);
List<Person> list = query.getResultList();
em.close();
return list;
}
public void save(String personName, Double personAge, String carName) {
Person person = new Person(personName, personAge);
Car car = new Car(carName);
car.setOwner(person);
person.getCars().add(car);
em.persist(person);
}
}
These two methods get called by the controller when clicking the corresponding buttons on the view.