I'm writing this post hoping it will be indexed and made available to OpenCart community.
If you have moved your OpenCart installation and checked official documentation along the way, you might have found it over simplistic. You just have to move the files between servers, import the database then change parameters in both config.php files, one inside root, the other inside /admin.
If you took those 3 steps and got an error 500 from your servers, in most of the cases you mistyped something in any of those config.php files. Once you mix them, your shop would be up and running.
If you have tripled checked all those paths and even so your shop is down, maybe there is something else to be done.
In my case, the "public" pages were fine, accessible, but I couldn't access the Admin site. I got multiple HTTP 500 errors. Checked my server error log, but it was clean, no lines at all.
So then I realized maybe the Admin folder could have some sort of access restriction. And it was exactly the case. Once I changed the folder access permissions to 755 -- bang! -- everything went back into place and I could normally access my admin login page.
Se ao mover uma instalação do OpenCart entre servidores ou para um novo domínio você receber erro 500 do servidor ao acessar a pasta /admin, talvez valha a pena verificar se as permissões de acesso à pasta estão definidas como 755 (Linux). No meu caso não estavam, e apesar dos dois arquivos config.php configurados corretamente, eu continuava tendo esse tipo de erro.