I'll give it a go.

I had forgotten to mention that it was an FTP connection and the script is generated with environment variables and the creation process had not changed for months, not had the value of the variables.

I'll see if it makes a difference.

Can you please try 5.0.5 beta?

As per bug 674 but in WinSCP 4.3.5 build 1463 using automation on XP SP3 which has worked fine for months;
Assertion failed: xdrPtr && xdrPtr == *xdrLPP, file xx.cpp, line 2233
Script:
==========
option batch on
option confirm off
option transfer binary
option exclude "thumbs.db; .ds_store;"
open -passive ftp://%FTP_USERNAME%:%FTP_PASSWORD%@%FTP_Srv%
option reconnecttime 10
synchronize remote \\Docs\tobeuploaded$\ProcUpload\Staging\ /Vendors
close
exit
==========
command line:
%FTP_Pth%\WinSCP\winscp.com /console /script=%FTP_Script% /log=%FTP_TmpLog%

The connection got lost and timed out a number of times during the upload.

Brian