This issue seems to affect 184.108.40.206 and the latest 220.127.116.11 64bit versions of MMEX running on Windows 7 64bit SP1.
If you are editing an existing account entry or creating a new one and begin typing the Payee reference the system correctly displays the list of existing payees that match the entered characters. If I spot one I want to use and use the arrow keys to highlight it and then the Enter key select it, the system deletes all characters entered in the field and I then have to change my selection to re-populate the field. The only way to make a selection from the drop down list is either to use a mouse click or press the Space key. On previous versions, the Enter key could also be used to make this selection.
Screenshot 1 = New account entry with payee patially keyed and matching list displayed.
Screenshot 2 = Cursor moved to required entry using arrow keys.
Screenshot 3 = Pressed Enter to select entry and field data is cleared.
Hope you can fix this in a future build.