User was appending a dbf file but was never presented with the file selection dialog.

The label0.ini was corrupt or was storing an invalid value for the last used append file.

Search for label0.ini and rename it to label0.old. It will be in either the 'WINDOWS' or 'WINNT' folder. EASYLABEL will create a new label0.ini the next time it is started and everything should work again.