If the GUI displays a file in the process of being backed up, and Task Manager indicates that Ntbackup is not responding, you probably have an invalid file name AND you enabled detailed logging.

The invalid file name is generally the next file, alphabetically, in the same folder as the file being displayed in the GUI.

NOTE: If you End Task, you can NOT use the current media in a restore.

The best solution is to locate the invalid file name and fix it, but you can also convert to summary level logging.