Check to see if this is a BOM problem.
On every Windows system, text files such as transaction import data files can come in two main flavors: ANSI and Unicode. Modern Windows systems (the Windows NT branch) have a native Unicode approach, and older ones (the Windows 95 branch) have native ANSI setup. Unicode allows for more multilengual characters.
An Unicode text file can also have an invisible preamble, which is called the BOM (Byte Order Mark). An Unicode sensible application will read the file with no problem.
The Dynamics SL application is traditionally built strictly as an ANSI program. If you feed in a BOM-Unicode data file to the Transation Import, the first line will read [BOM]Batch instead of Batch and a 8021 error triggers.
To check if this is the case, open any data file with a text editor (Notepad will do) and then do a "Save As..." and make sure the encoding is set as ANSI (not Unicode, not UTF-8, not UTF-16). Then try again the import and see if it works. This would mean that the latest updates caused that now an Unicode enforcement is applied somewhere in the pipeline of your data files.
