The pages looks for old css and js files, thus site shows crashed

Hi,

That happens because you’re using a cache system that caches the whole page.

Not logged in users see that cache page until it expires or you purge the cache manually.
Logged in users never see cached pages (usually) so you always see the site as it is.

When you purge the cache on this plugin settings, it only purges it’s own cache.
If you use a cache plugin, or some server cache system such as godaddy, you need to purge their cache system in order for the front-end to refresh.

This info is on the faq questions at https://www.ads-software.com/plugins/fast-velocity-minify/#faq under the “Why is it that even though I have disabled or removed the plugin, the design is still broken?”