Server Error in '/' aplication ...

I have an application in aspnet 4.0 and SQLSERVER., Which occasionally get error:
Server Error in '/' aplication ...

My Hardware:

windows server 2008r2, x64
intel xenon - 2ghz
4 gigs of ram
have 17 gigs of disk space
average of 25 users in parallel;
This is the web server and sqlserver at once.
backup of the database weighs 200MB.

ONE OF THE ERRORS THAT I WENT WAS:

'System.Data.DataRowView' does not Contain a property with the name 'ContratoId'

I do not understand, as in all the pages ..... has sometimes get error,
has come back on the page, and I get all ok ....

Another mistake came out: Datasource null

I think my application dies (ie comes Server Error in '/' aplication) due to lack of disk space ....
because I have 31 GB of disk space ....

and as I have read the windows server 2008r2 requires minimum 32 GB of HD:

http://gerardoramosun.wordpress.com/2010/07/09/instalando-y-configurando-windows-server-2008-r2-y-sql-server-2008-r2/

I have about 30 users in parallel, and this link dedusco missing HD space ..

http://www.htmlpoint.com/iis/02.htm