If the error 500 occurs, this error is displayed directly in the browser, but a completely white WordPress page is also possible. In both cases a login to the backend is no longer possible. If the error occurs shortly after a PHP change, the following solution can be helpful.
Solution: Mostly it is plugins that produce an error due to the PHP changeover, which goes beyond a simple error message and thus disturbs the entire system.
In the first step you should rename the folder /plugins/ to narrow down the error. To do this, access the server with an FTP program. If this is the reason, you should go through the plugins in order to localize the faulty plugin according to the exclusion criterion.