The solution is simple!
protip: read the logs
This was in my log.
2 NO [ALERT] Application-specific password required: https://support.google.com/accounts/answer/185833 (Failure)
Basically you have to create an app specific password for Mailspring. Go to this link (Create and manage your app passwords - https://myaccount.google.com/apppasswords ) & create a new app password specific for mailspring. Name the app password whatever you want. Go nuts and name it something like “Mailspring” then you’ll be prompted with that one time password. Copy it with the spaces and everything included.
Now in Mailspring click the Reconnect button in red that you’re prompted with. It’ll give you a link and / or open a new browser window. Don’t connect that way, instead in the mailspring prompt with the link it provides hit the back button and reconnect the account using IMAP. In the Password field you would enter the password you created for the app from the link above. The account should reconnect after those credentials are entered. Just make sure you change the SMTP port to 465.
I hope this helps! 