Hi experts, any help appreciated!

An ASP-Net-WebService reads Data from an Access-mdb via NHibernate.
Normally all works fine, the WebService is called, reads from the database, does its work and returns the data.

But when use MS-Access to edit some data and call the WebService with Access still open the WebService throws the following exception:

2010-10-25 16:13:29 DEBUG NHibernate.Util.ADOExceptionReporter - SQL Exception
System.Data.OleDb.OleDbException: Could not use ''; file already in use.
at System.Data.OleDb.OleDbConnectionInternal..ctor(OleDbConnectionString constr, OleDbConnection connection)
at System.Data.OleDb.OleDbConnectionFactory.CreateConnection(DbConnectionOptions options, Object poolGroupProviderInfo, DbConnectionPool pool, DbConnection owningObject)
at System.Data.ProviderBase.DbConnectionFactory.CreateNonPooledConnection(DbConnection owningConnection, DbConnectionPoolGroup poolGroup)
at System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection)
at System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory)
at System.Data.OleDb.OleDbConnection.Open()
at NHibernate.JetDriver.JetDbConnection.Open()
at NHibernate.Connection.DriverConnectionProvider.GetConnection()
at NHibernate.AdoNet.ConnectionManager.GetConnection()
at NHibernate.AdoNet.AbstractBatcher.Prepare(IDbCommand cmd)
at NHibernate.AdoNet.AbstractBatcher.ExecuteReader(IDbCommand cmd)
at NHibernate.Loader.Loader.GetResultSet(IDbCommand st, Boolean autoDiscoverTypes, Boolean callable, RowSelection selection, ISessionImplementor session)

Here is the section of the web.config file:
<session-factory>
<property name="connection.provider">NHibernate.Connection.DriverConnectionProvider</property>
<property name="dialect">NHibernate.JetDriver.JetDialect, NHibernate.JetDriver</property>
<property name="connection.driver_class">NHibernate.JetDriver.JetDriver, NHibernate.JetDriver</property>
<property name="connection.connection_string">Provider=Microsoft.Jet.OLEDB.4.0;Data Source=[Path to my mdb]</property>
<property name='proxyfactory.factory_class'>NHibernate.ByteCode.Castle.ProxyFactoryFactory, NHibernate.ByteCode.Castle</property>
</session-factory>

MS-Access is configured to open files shared, so opening and editing the mdb with two instances of MS-Access works fine.

The WebService also works fine when I open the mdb with MS-Access in shared read-only-mode.

Therefore I think NHibernate tries to force an exclusive open.
How can I tell NHibernate to open the mdb shared?
Or is there another solution?

Thank you for your time (and hopefully for your help)!
Martin