The same thing happens to me, the only error that I have the console is this:
application-e3e2b65734231ede09b9034749bc29d929feb55a4b1dd4f0e9cab9440c643ac4.js:sourcemap:1 Uncaught TypeError: e._perFileData is not a function
at application-e3e2b65734231ede09b9034749bc29d929feb55a4b1dd4f0e9cab9440c643ac4.js:sourcemap:1:622332
at Array.forEach (<anonymous>)
at Object.onBeforeUpload (application-e3e2b65734231ede09b9034749bc29d929feb55a4b1dd4f0e9cab9440c643ac4.js:sourcemap:1:622286)
at H.upload (vendor-96a5eaa8d436066b954c21ae5afb7f504310945c679465a2f1ba46b429a95edf.js:sourcemap:77:151420)
at vendor-96a5eaa8d436066b954c21ae5afb7f504310945c679465a2f1ba46b429a95edf.js:sourcemap:77:154902
I rebuilt several times and even tried in Safe mode thinking it might be a plugin or theme