There must be a problem with your installation because that file should be present there
webmail.zebrando.com/:1 Refused to apply style from 'https://webmail.zebrando.com/snappymail/snappymail/v/2.15.3/static/css/app.min.css' because its MIME type ('text/html') is not a supported stylesheet MIME type, and strict MIME checking is enabled.
/snappymail/snappymail/v/2.15.3/static/js/libs.js:1 Failed to load resource: the server responded with a status of 404 ()
webmail.zebrando.com/:1 Refused to execute script from 'https://webmail.zebrando.com/snappymail/snappymail/v/2.15.3/static/js/libs.js' because its MIME type ('text/html') is not executable, and strict MIME type checking is enabled.
(index):73 Uncaught (in promise) Error: Failed loading /snappymail/snappymail/v/2.15.3/static/js/libs.js
at script.onerror ((index):73:34)
/snappymail/snappymail/v/2.15.3/static/apple-touch-icon.png:1 Failed to load resource: the server responded with a status of 404 ()
manifest.json:1 Failed to load resource: the server responded with a status of 404 ()
manifest.json:2 Manifest: Line: 2, column: 1, Syntax error.
webmail.zebrando.com/:1 Refused to apply style from 'https://webmail.zebrando.com/snappymail/snappymail/v/2.15.3/static/css/app.min.css' because its MIME type ('text/html') is not a supported stylesheet MIME type, and strict MIME checking is enabled.
manifest.json:1 Failed to load resource: the server responded with a status of 404 ()
How did you manage to change your installation like this?