The program fails to import data once the database has been cleaned up.
Removing redundant records from the database is necessary for keeping the database size within reasonable range. When the database is cleaned up, all its records are deleted. However, besides the records themselves the database also stores the information that the log file had already been imported, and in the future this file's information should be imported not completely but rather from the point the last import process stopped at. This information is not deleted during the database cleanup. Therefore, during the next import the program "thinks" the log file had already been imported and does not import it again. As a result, the data imported during the previous importing session will not be imported again. To solve the problem: completely clean up the database (remove all the database files) and import the log files once again.