After running for some days without problems I just get NullPointerExceptions for every operation.
I restarted my application (not the server) and the mongo-driver worked again.

The first error:
java.lang.NullPointerException: null
at com.mongodb.DBTCPConnector$MyPort.get(DBTCPConnector.java:274) ~[mongo-java-driver-2.3.jar:na]
at com.mongodb.DBTCPConnector.call(DBTCPConnector.java:205) ~[mongo-java-driver-2.3.jar:na]
at com.mongodb.DBApiLayer$MyCollection.__find(DBApiLayer.java:284) ~[mongo-java-driver-2.3.jar:na]
at com.mongodb.DB.command(DB.java:152) ~[mongo-java-driver-2.3.jar:na]
at com.mongodb.DB.command(DB.java:141) ~[mongo-java-driver-2.3.jar:na]
at com.mongodb.DB.getLastError(DB.java:302) ~[mongo-java-driver-2.3.jar:na]

and afterwards for example:

Caused by: java.lang.NullPointerException: null
at com.mongodb.DBTCPConnector$MyPort.get(DBTCPConnector.java:274) ~[mongo-java-driver-2.3.jar:na]
at com.mongodb.DBTCPConnector.call(DBTCPConnector.java:205) ~[mongo-java-driver-2.3.jar:na]
at com.mongodb.DBApiLayer$MyCollection.__find(DBApiLayer.java:284) ~[mongo-java-driver-2.3.jar:na]
at com.mongodb.DB.command(DB.java:152) ~[mongo-java-driver-2.3.jar:na]
at com.mongodb.DBCollection.getCount(DBCollection.java:642) ~[mongo-java-driver-2.3.jar:na]
at com.mongodb.DBCollection.getCount(DBCollection.java:616) ~[mongo-java-driver-2.3.jar:na]
at com.mongodb.DBCursor.count(DBCursor.java:555) ~[mongo-java-driver-2.3.jar:na]

java.lang.NullPointerException: null
at com.mongodb.DBTCPConnector$MyPort.get(DBTCPConnector.java:274) ~[mongo-java-driver-2.3.jar:na]
at com.mongodb.DBTCPConnector.say(DBTCPConnector.java:151) ~[mongo-java-driver-2.3.jar:na]
at com.mongodb.DBTCPConnector.say(DBTCPConnector.java:141) ~[mongo-java-driver-2.3.jar:na]
at com.mongodb.DBApiLayer$MyCollection.update(DBApiLayer.java:317) ~[mongo-java-driver-2.3.jar:na]
at com.mongodb.DBCollection.update(DBCollection.java:112) ~[mongo-java-driver-2.3.jar:na]