Query causes an nternal exeption

Do know why we get this exception?

Caused by: com.objectdb.o.InternalException: Unexpected internal exception
    at com.objectdb.o.JPE.e(JPE.java:170)
    at com.objectdb.o.ERR.h(ERR.java:66)
    at com.objectdb.o.OBC.onObjectDBError(OBC.java:1591)
    at com.objectdb.jpa.JpaQuery.getResultList(JpaQuery.java:738)
    ... 3 common frames omitted
Caused by: java.lang.NullPointerException: null
    at com.objectdb.o.ENH.b(ENH.java:414)
    at com.objectdb.o.OBC.ai(OBC.java:976)
    at com.objectdb.o.OBC.Zc(OBC.java:800)
    at com.objectdb.o.SRB.a(SRB.java:162)
    at com.objectdb.o.QRR.h(QRR.java:565)
    at com.objectdb.o.QRR.i(QRR.java:225)
    at com.objectdb.jpa.JpaQuery.getResultList(JpaQuery.java:726)
    ... 22 common frames omitted

Could you provide more information about the relevant executed query, and particularly about the expected result objects?

It seems that a query result has an unexpected type (maybe an embedded object)?

The reason was missing entity classes in the classpath during migration of the database to the next version of our tool.
After that the missing entities can not be found anymore by queries.

Thank you for your support.