Data Read / Write Problem With Concurrent Users On With Dll

Hi,
I have a web app, that runs fine, except for one particular section that uses a class called by an event in the code behind. The class resides as a dll in the bin folder.
 
We had no problems during testing, when only one user was running this dll. Problems soon occurred when multiple users tried running it. Here's the error & stack:
 
06/02/2007 09:25:26 ==> cburns ==> There is already an open DataReader associated with this Command which must be closed first.   at System.Data.SqlClient.SqlInternalConnectionTds.ValidateConnectionForExecute(SqlCommand command)
   at System.Data.SqlClient.SqlConnection.ValidateConnectionForExecute(String method, SqlCommand command)
   at System.Data.SqlClient.SqlCommand.ValidateCommand(String method, Boolean async)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult result)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method)
   at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior, String method)
   at System.Data.SqlClient.SqlCommand.ExecuteReader()
   at ESP.Validator.Data.DatabaseEvents.DatabaseEventManager.Read(IEventable eventObject, Int16 eventType, DateTime earliestDate, DateTime latestDate) in C:My PathValidatorValidator.NETDataDatabaseEventsDatabaseEventManager.cs:line 92
   at ESP.Validator.Data.Translink.CATCard.GetDespatchDate() in C:My PathProjectsValidatorValidator.NETDataTranslinkCATCard.cs:line 94
   at ESP.Validator.Data.Translink.ExistingSchemeEntitlement.ReadCards() in C:My PathProjectsValidatorValidator.NETDataTranslinkExistingSchemeEntitlement.cs:line 215
   at ESP.Validator.Data.Translink.ExistingSchemeEntitlement.Read() in C:My PathProjectsValidatorValidator.NETDataTranslinkExistingSchemeEntitlement.cs:line 147
   at ESP.Validator.Data.Translink.TranslinkApplicant.ReadEntitlements() in C:My PathProjectsValidatorValidator.NETDataTranslinkTranslinkApplicant.cs:line 369
   at ESP.Validator.Data.Translink.TranslinkApplicant.Read() in C:My PathProjectsValidatorValidator.NETDataTranslinkTranslinkApplicant.cs:line 353
   at ESP.Validator.Data.Translink.PrePrintedLetter.Read() in C:My PathProjectsValidatorValidator.NETDataTranslinkPrePrintedLetter.cs:line 282
   at ESP.Validator.ValidationProcessor.Read(ValidationSubject subject) in C:My PathProjectsValidatorValidator.NETValidationProcessor.cs:line 82
   at clear_applications_scan_applications.ProcessValidation()
 
It seems the data reader is getting reused. We have ensured after each read the reader is closed. Though all users are using the same connection string. Could it be a connection pool problem, with the connection being overwritten during execution? Should i edit it according to the users logon?
I am really at a loss for ideas, and I don't mind admitting I am a bit out of my depth with this one!!
 Any ideas/suggestions would be greatly appreciated.
 
Thanks