Starting with POPFile 0.22 only critical errors are logged by default. For debugging it is often useful to see full details of what is going on so if you can get the problem to repeat please submit a log with the logger level turned up to 2.
You can do this on the Advanced page by changing ' logger_level' from the default 0 to 2. You must then restart POPFile for the new logger level to take effect. Also make sure that logging is enabled on the Configuration Page.
You can always find the current log file if you follow the link on the Configuration page.