Internal connection fatal error and other random db connection err

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Hi guys!

My newly built website has just recently went online,

but sometimes I face errors (randomly) like these:

System.IndexOutOfRangeException: There is no row at position 0.
System.InvalidOperationException: There is already an open DataReader
associated with this Command which must be closed first.
System.IndexOutOfRangeException: Cannot find table 0

System.NullReferenceException: Object reference not set to an instance of an
object.

System.InvalidOperationException: ExecuteNonQuery requires an open and
available Connection. The connection's current state is closed.

System.InvalidOperationException: Internal connection fatal error.
..
..
..
ExecuteReader

I use a dbmanager class as a wrapper for db ,

which I uploadere here:

http://mihd.net/9k47re

The strange thing is that both iis and the database lie on the same machine,

but I use tcp/ip for incoming connections,

so,

could anyone help me get rid of these errors somehow?

thanks in advance!