Transaction problem with Access 97 under Windows 98SE

Hi all:

The program I wrote just produced an error message on one of the client's
computers. A little background: I installed MDAC 2.6, Jet SP4 and Jet update
on all of the computers - same updates in the same order. The machines are
the same: Windows 98SE updated to the latest possible level from the Windows
Update - nothing left to install.

Only one of those machines is giving an error (quoting by memory) 'Unable to
commit transaction. BeginTrans has not been called' in the code like that:

if AdoDataSet1.FieldByName('OrderPosted').AsBoolean = False then
begin
  AdoConnection1.BeginTrans;
  try
      AdoConnection1.Execute('UPDATE ... SET ...'); // <- multiple lines
like this

      AdoDataSet1.Edit;
      AdoDataSet1.FieldByName('OrderPosted').AsBoolean = True;
      AdoDataSet1.Post;

      AdoConnection1.CommitTrans; // <- The exception occurs here
  except
      AdoConnection1.RollbackTrans;
      MessageDlg( meaningful message );
  end;
end;

The other computers execute this portion of code no problem. Running MS Com
Checker did not produce any valuable information: the MDAC and JET DLL
versions are the same everywhere in the client's network. The app is D5
running over Jet 4.0 against the Access 97 database.

Any ideas welcome.

Alex