Try changing the order of the two action filters on the WebApi action methods that return an IQueryable:
[ DateConversionFilter(DateConversionFilterOperation.DeclareLocale), Queryable]
There is a Bug in WebApi for which the order of execution of attributes cannot be controlled in a deterministic way, so if you modify somehow the code or update to a newer version of a nuget package the order of execution of action filters changes. So in
order to fix you have to reorder the them in the code....very annoying. See here: http://www.strathweb.com/2012/06/control-the-execution-order-of-your-filters-in-asp-net-web-api/ for
more infos on this problem.
If Microsoft doesnt fix this problem in the next release of the Mvc Controls Toolkit I will provide a solution similar to the one described in that link.