Starting with POPFile v0.22, you can adjust the logging level. You can find it on the Advanced Tab; the variable's name is 'logger_level'. The default setting is 0 which will only log critical errors, 1 is much more verbose, and you get to see everything if it's set to 2. If you are debugging a problem setting to 2 is probably necessary.

The log file can be found in your POPFile user directory. You can also view it from the Configuration tab, the Logging section has a link to “Download current log file.”