I have some additional code that might help. I'll dig it out.

What code do you have for outputting the Sage errors already?

I was using Session.Errors but that is empty in this case, other then that just a try/catch. I see they return a code '6666', but there's no reference as to what that means. So, I was wondering if they have the codes listed somewhere

In addition to Vega's questions, have you composed your views?  In your example you only show opening OE0520 - there are close to a dozen views necessary to create an order entry order.

Also - check the customer's credit limit or if they are on hold.  The OE Order Header view will return 'odd numbers' in the error code to relay information back to you.

Also search for 6666 on this old page:  https://smist08.wordpress.com/about/

I think you have something fundamentally wrong with your customization.

I use this to catch session errors:

            try
            {
              // Your code
            }
            catch(SessionException ex)
            {
                HandleLogRequest($"In {ex.GetType().Name} handler");
                HandleLogRequest("ex.Reason: " + ex.Reason.ToString());

                if (ex.Data.Count > 0)
                {
                    foreach (System.Collections.DictionaryEntry de in ex.Data)
                    {
                        HandleLogRequest($"Key: {de.Key.ToString()}      Value: {de.Value}");
                    }
                }
                else
                {
                    HandleLogRequest("ex.Data.Count was 0");
                }

                HandleLogRequest("ex.GetBaseException(): " + ex.GetBaseException().ToString());
                HandleLogRequest("ex.HResult: " + ex.HResult.ToString());
                HandleLogRequest("ex.InnerException: " + ex.InnerException.ToString());
                HandleLogRequest("ex.Message: " + ex.Message.ToString());
                HandleLogRequest("ex.Source: " + ex.Source.ToString());
                HandleLogRequest("ex.StackTrace: " + ex.StackTrace.ToString());
                HandleLogRequest("ex.TargetSite: " + ex.TargetSite.ToString());
                HandleLogRequest("WriteSageErrorsToLog(): " + WriteSageErrorsToLog());

                iReturnCode = (int)ExitCodes.CannotLogIntoSage;
            }
            catch (Exception ex)
            {
                HandleLogRequest($"In {ex.GetType().Name} handler");

                if (ex.Data.Count > 0)
                {
                    foreach (System.Collections.DictionaryEntry de in ex.Data)
                    {
                        HandleLogRequest($"Key: {de.Key.ToString()}      Value: {de.Value}");
                    }
                }
                else
                {
                    HandleLogRequest("ex.Data.Count was 0");
                }

                HandleLogRequest("ex.GetBaseException(): " + ex.GetBaseException().ToString());
                HandleLogRequest("ex.HResult: " + ex.HResult.ToString());
                HandleLogRequest("ex.InnerException: " + ex.InnerException.ToString());
                HandleLogRequest("ex.Message: " + ex.Message.ToString());
                HandleLogRequest("ex.Source: " + ex.Source.ToString());
                HandleLogRequest("ex.StackTrace: " + ex.StackTrace.ToString());
                HandleLogRequest("ex.TargetSite: " + ex.TargetSite.ToString());
                HandleLogRequest("WriteSageErrorsToLog(): " + WriteSageErrorsToLog());

                iReturnCode = (int)ExitCodes.CannotLogIntoSage;
            }

And the method to write out the Sage errors:

        private string WriteSageErrorsToLog()
        {
            if (mbDebugMode) HandleLogRequest($"In {MethodBase.GetCurrentMethod().Name}()");

            string sError = "";

            if (accpacSession != null)
            {
                if (accpacSession.Errors != null)
                {
                    for (int i = 0; i < accpacSession.Errors.Count; i++)
                    {
                        LogHelper.Log(LogTarget.EventLog, $"Sage Error: {accpacSession.Errors[i].Message}");
                        HandleLogRequest($"Error number {i.ToString()}: Sage error code: {accpacSession.Errors[i].Code} - Message: {accpacSession.Errors[i].Message} - Source: {accpacSession.Errors[i].Source}");
                        sError += "\r\n" + accpacSession.Errors[i].Message;
                    }

                    accpacSession.Errors.Clear();
                }
            }
            else
            {
                HandleLogRequest("No session available");
            }

            return sError;
        }

My log helper is a class I wrote that can dump out to a log file, event log, or a database if needed.