Time to time we have problem

Could not compile the mapping document: Domain.Mapping.Resource.hbm.xml NHibernate.MappingException: Could not compile the mapping document: Domain.Mapping.Resource.hbm.xml ---> System.Xml.XPath.XPathException: Namespace prefix 'hbm' is not defined.

at MS.Internal.Xml.XPath.CompiledXpathExpr.UndefinedXsltContext.LookupNamespace(String prefix)
at MS.Internal.Xml.XPath.BaseAxisQuery.SetXsltContext(XsltContext context)
at MS.Internal.Xml.XPath.CompiledXpathExpr.SetContext(XmlNamespaceManager nsManager)
at System.Xml.XmlNode.SelectNodes(String xpath, XmlNamespaceManager nsmgr)
at NHibernate.Cfg.HbmBinder.GetMetas(XmlNode node)
at NHibernate.Cfg.HbmBinder.BindProperty(XmlNode node, Property property, Mappings mappings)
at NHibernate.Cfg.HbmBinder.CreateProperty(IValue value, String propertyName, Type parentClass, XmlNode subnode, Mappings mappings)
at NHibernate.Cfg.HbmBinder.PropertiesFromXML(XmlNode node, PersistentClass model, Mappings mappings)
at NHibernate.Cfg.HbmBinder.BindRootClass(XmlNode node, RootClass model, Mappings mappings)
at NHibernate.Cfg.HbmBinder.BindRoot(XmlDocument doc, Mappings mappings)
at NHibernate.Cfg.Configuration.AddValidatedDocument(XmlDocument doc, String name)

any suggestion about this problem ?

ASP.NET 2.0, Entity as separate DLL .
SysCache2 in use + MSSQL 2005