This can happen when the URL initially uses https, but after you log in, Mosaic tries to load using http instead (missing the "S"), which causes it to freeze and display an error.
To check if this is the issue, try manually adding https to the address bar and pressing Enter. While this isn't a permanent fix, it can confirm the cause.
If that works, please raise a new case online and reference this article's title. Make sure to include a screenshot of the error with the full address bar visible at the top.