I saw this issue #299
looks like similar, but marked as resolved.
I use last version 5.0.3

java.lang.ClassCastException: com.test.CategoryTableModelItem$ExpandableHeaderViewHolder cannot be cast to com.test.SubcategoryTableModelItem$ExpandableHeaderViewHolder
at com.test.SubcategoryTableModelItem.bindViewHolder(SubcategoryTableModelItem.java:32)
at eu.davidea.flexibleadapter.FlexibleAdapter.onBindViewHolder(FlexibleAdapter.java:1769)
at eu.davidea.flexibleadapter.FlexibleAdapter.onBindViewHolder(FlexibleAdapter.java:1746)
at eu.davidea.flexibleadapter.helpers.StickyHeaderHelper.updateHeader(StickyHeaderHelper.java:168)
at eu.davidea.flexibleadapter.helpers.StickyHeaderHelper.updateOrClearHeader(StickyHeaderHelper.java:139)
at eu.davidea.flexibleadapter.FlexibleAdapter$AdapterDataObserver$1.run(FlexibleAdapter.java:5282)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)