Yep, the easiest way is through your inaccesible web-based admin page but equally you can go into the configuration file and change the value there.
You are looking for a file called firefly.conf and within that your settings are stored. (Sorry, not sure where Windows stores that but a search should turn it up quickly) Scroll down til you see the section on the Debug level and you’ll see the value which is probably set at 1 currently. Just change that to 5 (for a goodly amount of logfile) or 9 for the whole shabang. Save that file, quit & restart firefly and you should now see the log splurging out a ream of stuff. Post that output and we can see what’s going on.
(Whilst you’re in the config file, I’d suggest looking at what your admin password is for accessing the web-admin. By default it should be mt-daapd but you may want to change it to something easier. When you try to log in to the webpage you shouldn’t need a username, just that password.)