My solution includes 2 ASP.NET project:
- user interface pages (1)
- Web Service (2)

Both projects uses NHibernate.
Project (1) includes wrapper class session.cs .
It create configuration, session factory and session.

Example of one of method of this class is:
private static ISessionFactory Factory
{
get
{
if( _factory == null )
{
Configuration config = SessionManager.Configuration;
lock( _locker )
{
if( _factory == null )
_factory = config.BuildSessionFactory();
}
}
return _factory;
}
}

I access NHibernate session in both projects
through this class.

Project (2) contains reference to project (1)
(it needs it to access session.cs class)

The problem is very interesting.

If and only if I enable second-level cache
in nhibernate settings in web.config of project (1)
the following error appears here
"_factory = config.BuildSessionFactory();"
when I go to web service:

could not instantiate CacheProvider: NHibernate.Caches.SysCache.SysCacheProvider, NHibernate.Caches.SysCache --> Specified cast is not valid.

What is the problem?

Can you post the whole exception information?
You may also try to use this cache in a simpler situation to isolate the problem...

NHibernate.HibernateException: could not instantiate CacheProvider: NHibernate.Caches.SysCache.SysCacheProvider, NHibernate.Caches.SysCache ---> System.InvalidCastException: Specified cast is not valid.
at NHibernate.Cfg.SettingsFactory.BuildSettings(IDictionary properties)
--- End of inner exception stack trace ---
at NHibernate.Cfg.SettingsFactory.BuildSettings(IDictionary properties)
at NHibernate.Cfg.Configuration.BuildSessionFactory()

In case of using SysCache for only one ASP.NET project
all worked fine.

By the way for some time the problem dissapeared when I
replaced NHibernate (and all other dlls it uses) for release 1.0.0.
But at the moment I see the same error.

Check that you use matching versions of NHibernate and NHibernateContrib dlls.