Cool app idea! I listen to a lot less classical music than I would like exactly due to the problem you are trying to solve.

I have the loading screen problem running the linux version aswell.
Running the app with logging enabled gives the following output. Hopefully this helps you with debugging.

λ ~/ export ELECTRON_ENABLE_LOGGING=1 && /opt/Concertmaster/concertmaster --remote-debugging-port=8315
[20025:1213/104849:INFO:CONSOLE(1)] "Uncaught SyntaxError: Unexpected token )", source: /opt/Concertmaster/resources/app.asar/js/jquery.js (1)
[20025:1213/104849:INFO:CONSOLE(1)] "Uncaught SyntaxError: Unexpected token )", source: /opt/Concertmaster/resources/app.asar/js/jquery.js (1)
[20025:1213/104849:INFO:CONSOLE(1)] "Uncaught SyntaxError: Unexpected string", source: file:///opt/Concertmaster/resources/app.asar/js/jquery.mousewheel.js (1)
[20025:1213/104849:INFO:CONSOLE(1)] "Uncaught SyntaxError: Unexpected token =", source: file:///opt/Concertmaster/resources/app.asar/js/jquery.leanModal.min.js (1)
[20025:1213/104849:INFO:CONSOLE(1)] "Uncaught SyntaxError: Invalid or unexpected token", source: file:///opt/Concertmaster/resources/app.asar/js/html.sortable.0.1.3.js (1)
[20025:1213/104849:INFO:CONSOLE(2)] "Uncaught SyntaxError: Unexpected token }", source: file:///opt/Concertmaster/resources/app.asar/js/toggles.min.js (2)
[20025:1213/104849:INFO:CONSOLE(1)] "Uncaught SyntaxError: Illegal return statement", source: file:///opt/Concertmaster/resources/app.asar/js/select2/js/select2.js (1)
[20025:1213/104849:INFO:CONSOLE(96)] "Uncaught SyntaxError: Error parsing /opt/Concertmaster/resources/app.asar/node_modules/@exponent/electron-cookies/package.json: Unexpected token e in JSON at position 2", source: module.js (96)
[20025:1213/104849:INFO:CONSOLE(1)] "Uncaught SyntaxError: Unexpected token )", source: file:///opt/Concertmaster/resources/app.asar/js/lib.js (1)
[20025:1213/104849:INFO:CONSOLE(41)] "Uncaught ReferenceError: request is not defined", source: file:///opt/Concertmaster/resources/app.asar/index.html (41)
[20025:1213/104849:INFO:CONSOLE(0)] "Failed to decode downloaded font: file:///opt/Concertmaster/resources/app.asar/css/PetitaMedium.ttf", source: file:///opt/Concertmaster/resources/app.asar/loader.html (0)
[20025:1213/104849:INFO:CONSOLE(0)] "OTS parsing error: invalid version tag", source: file:///opt/Concertmaster/resources/app.asar/loader.html (0)
[20025:1213/104849:INFO:CONSOLE(0)] "Failed to decode downloaded font: file:///opt/Concertmaster/resources/app.asar/css/Sanchezregular.otf", source: file:///opt/Concertmaster/resources/app.asar/loader.html (0)
[20025:1213/104849:INFO:CONSOLE(0)] "OTS parsing error: invalid version tag", source: file:///opt/Concertmaster/resources/app.asar/loader.html (0)
[20025:1213/104849:INFO:CONSOLE(153)] "Uncaught ReferenceError: $ is not defined", source: file:///opt/Concertmaster/resources/app.asar/index.html (153)
[20025:1213/104849:INFO:CONSOLE(0)] "Failed to decode downloaded font: file:///opt/Concertmaster/resources/app.asar/css/PetitaMedium.ttf", source: file:///opt/Concertmaster/resources/app.asar/index.html (0)
[20025:1213/104849:INFO:CONSOLE(0)] "OTS parsing error: invalid version tag", source: file:///opt/Concertmaster/resources/app.asar/index.html (0)
[20025:1213/104849:INFO:CONSOLE(0)] "Failed to decode downloaded font: file:///opt/Concertmaster/resources/app.asar/css/Sanchezregular.otf", source: file:///opt/Concertmaster/resources/app.asar/index.html (0)
[20025:1213/104849:INFO:CONSOLE(0)] "OTS parsing error: invalid version tag", source: file:///opt/Concertmaster/resources/app.asar/index.html (0)
[20025:1213/104849:INFO:CONSOLE(284)] "Uncaught ReferenceError: spotinit is not defined", source: file:///opt/Concertmaster/resources/app.asar/index.html (284)
[20025:1213/104849:INFO:CONSOLE(347)] "Uncaught ReferenceError: $ is not defined", source: file:///opt/Concertmaster/resources/app.asar/index.html (347)
[20025:1213/104849:INFO:CONSOLE(360)] "Uncaught ReferenceError: $ is not defined", source: file:///opt/Concertmaster/resources/app.asar/index.html (360)

System info:

λ ~/ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 16.04.3 LTS
Release: 16.04
Codename: xenial
λ ~/ uname -a
Linux thom-laptop 4.4.0-101-generic #124-Ubuntu SMP Fri Nov 10 18:29:59 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
λ ~/