Affects Version/s: 1.15.1
Fix Version/s: 1.16.1
This is down to a race condition, I think.
At any rate, the ObjectSpecificationOnStandaloneList for FreeStandingList (the internal pojo that holds the result of an action invocation) is asked for its ObjectSpecId. It tries to do this by asking for the value of the corresponding ObjectSpecIfFacet.
However, no such facet (in some circumstances, presumably) is installed for this ObjectSpec, and so it searches up the hierarchy. The superclass is java.lang.Object, and so we return the ObjectSpecId for java.lang.Object.
This in turn causes the cache of ObjectSpecId -> ObjectSpecification's to be corrupted, with java.lang.Object mapping to the list.
In the Wicket viewer, when attempting to render a reference property of type java.lang.Object (such as the Task_object mixin for Estatio), the SpecificationLoader reports that a reference of this type is not isObject() (because it's using ObjectSpecificationOnStandaloneList), resulting in an UnknownPanel being selected to render the Wicket model for the reference property. This simply shows a toString().
The fix, I think, is to not search up the hierarchy in the case of ObjectSpecIdFacet... an ObjectSpecification must be able to answer this question itself.