If Marvin isn't loading for you, you can open the console to check for errors. If you see an error message like "IndexedDB gone bad", then it's time to clear application storage! This will clear all the data on this browser, but when you load Marvin again, the app will fetch all your data from the cloud as if you connected with this browser for the first time (Be patient! This could take a few minutes).
Once you've opened the console, find the Application tab:
Then choose the Storage tab on the left, and click the "Clear site data" button.
That's it! Now just reload the web page or restart the desktop app and you should be good to go.