NoClassDefFound within JPA when calling getResultList()

We're using JPA (eclipselink 2.5.2 and also tried 2.6.2) and are facing an odd issue:

We have some classes, which are accessed via a proxy of itself (as described in this post (the marked solution): Call a method any time other methods are called )

Now, we're facing the following issue: When calling getResultList() on our named queries, we now get this exception:

Caused by: java.lang.NoClassDefFoundError: Could not initialize class java.lang.reflect.Proxy$ProxyAccessHelper
at java.lang.reflect.Proxy.newProxyInstance(Proxy.java:746)
at com.sap.core.persistence.jdbc.trace.ResultSetProxy.createProxy(ResultSetProxy.java:27)
at com.sap.core.persistence.jdbc.trace.TraceablePreparedStatement.executeQuery(TraceablePreparedStatement.java:78)
at org.apache.commons.dbcp.DelegatingPreparedStatement.executeQuery(DelegatingPreparedStatement.java:96)
at org.apache.commons.dbcp.DelegatingPreparedStatement.executeQuery(DelegatingPreparedStatement.java:96)
at org.apache.commons.dbcp.DelegatingPreparedStatement.executeQuery(DelegatingPreparedStatement.java:96)
at org.eclipse.persistence.internal.databaseaccess.DatabaseAccessor.executeSelect(DatabaseAccessor.java:1007)
at org.eclipse.persistence.internal.databaseaccess.DatabaseAccessor.basicExecuteCall(DatabaseAccessor.java:642)
at org.eclipse.persistence.internal.databaseaccess.DatabaseAccessor.executeCall(DatabaseAccessor.java:558)
at org.eclipse.persistence.internal.sessions.AbstractSession.basicExecuteCall(AbstractSession.java:1991)
at org.eclipse.persistence.sessions.server.ServerSession.executeCall(ServerSession.java:570)
at org.eclipse.persistence.sessions.server.ClientSession.executeCall(ClientSession.java:250)
at org.eclipse.persistence.internal.queries.DatasourceCallQueryMechanism.executeCall(DatasourceCallQueryMechanism.java:242)
at org.eclipse.persistence.internal.queries.DatasourceCallQueryMechanism.executeCall(DatasourceCallQueryMechanism.java:228)
at org.eclipse.persistence.internal.queries.DatasourceCallQueryMechanism.executeSelectCall(DatasourceCallQueryMechanism.java:299)
at org.eclipse.persistence.internal.queries.DatasourceCallQueryMechanism.selectAllRows(DatasourceCallQueryMechanism.java:694)
at org.eclipse.persistence.internal.queries.ExpressionQueryMechanism.selectAllRowsFromTable(ExpressionQueryMechanism.java:2738)
at org.eclipse.persistence.internal.queries.ExpressionQueryMechanism.selectAllRows(ExpressionQueryMechanism.java:2691)
at org.eclipse.persistence.queries.ReadAllQuery.executeObjectLevelReadQuery(ReadAllQuery.java:495)
at org.eclipse.persistence.queries.ObjectLevelReadQuery.executeDatabaseQuery(ObjectLevelReadQuery.java:1168)
at org.eclipse.persistence.queries.DatabaseQuery.execute(DatabaseQuery.java:899)
at org.eclipse.persistence.queries.ObjectLevelReadQuery.execute(ObjectLevelReadQuery.java:1127)
at org.eclipse.persistence.queries.ReadAllQuery.execute(ReadAllQuery.java:403)
at org.eclipse.persistence.queries.ObjectLevelReadQuery.executeInUnitOfWork(ObjectLevelReadQuery.java:1215)
at org.eclipse.persistence.internal.sessions.UnitOfWorkImpl.internalExecuteQuery(UnitOfWorkImpl.java:2896)
at org.eclipse.persistence.internal.sessions.AbstractSession.executeQuery(AbstractSession.java:1793)
at org.eclipse.persistence.internal.sessions.AbstractSession.executeQuery(AbstractSession.java:1775)
at org.eclipse.persistence.internal.sessions.AbstractSession.executeQuery(AbstractSession.java:1740)
at org.eclipse.persistence.internal.jpa.QueryImpl.executeReadQuery(QueryImpl.java:258)
at org.eclipse.persistence.internal.jpa.QueryImpl.getResultList(QueryImpl.java:469)

I debugged into this for a long time to no avail. The jpa using method is in the same context as the proxy is defined, so basically the actual proxy class should exist.

If anyone has a clue, please enlighten me.

Thanks and regards, Kay