PROBLEM: You cannot get vivaldi browser to load after changing your computer’s hostname (by editing the name appearing in the etc/hostname file)? I was getting a non-responsive black screen until I found this solution in an old reddit post. This tidbit might be useful to you sometime…
It’s likely your vivaldi profile is now locked in connection with your previous hostname.
Fortunately, it’s fairly easy to remove the lock:
Find the various files whose names start with “Singleton…” in your ~/.config/vivaldi/ directory and simply delete them. New lock files will be generated (as needed) under your new hostname anyhow.
In my case, the (3) files looked like this:
Easy peasy. Vivaldi loads up again as before having renamed my computer.
[BTW, if you had quit vivaldi before rebooting, you will have avoided the lockfile problem entirely. But if so, you wouldn’t likely be reading this post now. 
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