Avoid MultipleBagFetchException when referencing multiple collections of another type

I know how to store a single list in a JPA class. but now everytime i try to add another list to any other entity class i get an ivy:error:persistence, with following error message:

IvyScriptRuntimeException: IvyScript Runtime Exception in
    Instruction:
        ivy.persistence.axsosUnit.findAll(teamprojekt.Job.class)
    Block:
        teamprojekt.showDashboard.showDashboardData out;
        out = in;
        in.data.closedJobs = ivy.persistence.axsosUnit.findAll(teamprojekt.Job.class);

IvyScriptMethodInvocationException: Error calling method findAll(java.lang.Class) on an object of class ch.ivyteam.ivy.process.data.persistence.internal.IvyEntityManager.

PersistenceUnitException: Failed to excecute operation "findAll" for persistence unit "axsosUnit".

PersistenceUnitException: Could not create EntityManagerFactory of persistence unit'axsosUnit'

PersistenceException: [PersistenceUnit: axsosUnit] Unable to build EntityManagerFactory

MultipleBagFetchException: cannot simultaneously fetch multiple bags

I have read, that you cant use two lists in the same entity class, but i use them in different entity classes. maybe because they are nested?

the structure is: institutionList is inside education which is inside application which has the other list.

I have also read, the solution is to use a set instead of a list. can you explain how to do this? I've tried to change the list to a set from the entity class wizzard, but without success.

can you show me how to do this?

thank you very much