What is your hosting company and plan?
THe error 500, internal server error is due to the server not being able to run a php script, for example, if the php version is too old, you ran out of memory, if we use a command that is either blocked or not supported, if the file permissions are wrong, if the server is misconfigured, etc.
You could ask the hosting company to check if they can do something about it.
There is however, a quick way to avoid the blank page.
You can select the option to disable the html minification on the FVM plugin (looks like your server doesn’t support it for some reason).
Now for the second part.
If you uninstall the plugin and the site doesn’t load, it’s probably a cache issue.
What happens is that some cheap hosting providers, cache a copy of your pages to make it feel faster. Then, when you change things around you cannot see the changes without purging the cache.
There should be a purge cache button somewhere either on your hosting control panel or on your wp-admin area. If it’s not, contact your hosting company and ask for it.
When you uninstall a plugin and even when you delete the files, the cache should refresh automatically, however, some hosting providers will cache files assuming things don’t change often… and that’s why, that even if you delete all the files, the server still reads them from memory (cache), “as if” those were still there.
If you have an option to restart the services (apache, php) or purge the cache, use it.
As for the error on your logs, can be a lot of things.
If you google a bit, you can see what I mean… but it’s mostly server related.
FVM works perfectly fine on most hosts, even on cheap providers such as godaddy and bluehost, so I’m curious to see what hosting you’re using.
Some info:
https://www.liquidweb.com/kb/apache-error-premature-end-of-script-headers/
https://stackoverflow.com/questions/17583341/error-500-premature-end-of-script-headers
https://support.plesk.com/hc/en-us/articles/213951565-PHP-FastCGI-mode-does-not-work-Premature-end-of-script-headers-
https://www.knownhost.com/wiki/developmental/what-is-the-premature-end-of-script-headers-php-error
Recommendation:
Make sure you are running “at least”, PHP 5.6 and that apache is updated.
Make sure all files are owned by the correct user for that site.
Else, disable HTML minification or try the “alternative html minification” option.
