We had a similar problem in one of our apps. The solution for us was to use the correct nhibernate types in the mapping.

Like you, our tables have Varchar. We noticed that hibernate was generating nvarchar for the data types.

Per the hibernate documentation, string will generate nvarchar in sql. The use of AnsiString will generate varchar parameter types and Sql Server is happy.

I'm on my smartphone at the moment, so I can't provide specifics. When I'm at a pc, I'll update with specific docs and code samples.