[SalesForce] CANNOT_EXECUTE_FLOW_TRIGGER Error

System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_EXECUTE_FLOW_TRIGGER, The record couldn’t be saved because it failed to trigger a flow. A flow trigger failed to execute the flow with version ID 301R0000000DGbx. Flow error messages: An unhandled fault has occurred in this flow
An unhandled fault has occurred while processing the flow. Please contact your system administrator for more information. Contact your administrator for help.: []

I am receiving the above error while running a test class which uses only custom metadata and no object .
The create user method in a util class where I am creating a test user is failing .

An error occurred at element myRule_1_A1 (FlowActionCall).
The flow failed to access the value for myVariable_current.CustomObject__r.Opportunity__r.Id because it hasn't been set or assigned.

P.S . I have checked several similar posts in the Forum which says that the fields used in flow should be set in the test class. But in my case , the objects aren't even used in my test class . The process builder failing runs on a different object.

How should I start solving?

Edit : [ User creation method which is failing]

public static User createUser(String profileName, Boolean isInsert){
    Profile p;

            list<Profile> listProfile = [SELECT Id FROM profile WHERE Name = :profileName];
            if(listProfile.size() > 0) {
            p = listProfile[0];
            } else {
                     return null;
                   }

           User testUser = new User(//user fields);
             if(isInsert){
                           insert testUser;
                         }

            system.assert(true);

    return testUser;
}