Strangely, Wifi sync stopped working again.
I made sure the network is set to Private on PC, and I even tried turning off the firewall (Comodo Security), but I kept getting the 'Unable to connect to Musicbee' error.
According to Wifi app's error log, it was trying to connect to 10.165.1.176 from 10.165.1.212, while my PC is now 10.165.2.20.
Musicbee.exe is added to the 'Trusted' list of Comodo security, and Windows firewall is disabled.
Musicbee's error log is empty so far.
Then, I tired deleting all the data of Wifi app from Android setting, and starting all over again.
However, even after running a wired sync from Musicbee, when pressing 'Locate Server', the wifi app shows 'Server not found'.
Now I don't have access to error log any more.
EDIT: Restarting PC & phone doesn't seem to help.