RE: Buisness Central Startup page not loading with finishing setup
If some other users are able to sign in, then the environment is most likely created correctly. What you should suspect is the PC or browser settings of the users who cannot sign in.
If using the browser's incognito mode solves the problem, then the problem is in the browser's settings or cache. There is a way to carefully clear the browser cache one by one, but it may require a deeper understanding of the browser.
In my experience, users with multiple tenants sometimes face this problem. My recommendation is to use Edge's Profile; I believe Chrome has a similar feature, and by using Edge's profile feature, you can completely eliminate the affect of tenants signed in in the past. Profiles also have the ability to remember IDs and passwords, so you don't have to type them in every time like you do in Chrome's Secret Mode or Edger's In Private Mode. Users can simply switch between profiles on the Edge for the tenant they want to use.