Thank you for contacting us and I'd be happy to assist.
I tried to log in to the website's admin area, but it keeps showing the 'not found' page.
The intermittent loading issues are mostly related to cached content. I'll recommend clearing all involved caches at the plugins, server, and CDN level and then check the page again.
For me, the page loads fine, in the couple of different browsers that I've tested.
As the missing styles from your screenshot, seems to be loaded through the Astra custom CSS add-on, I'll suggest reporting this to their official support, if this continues.
Even after logging in, I wasn't able to see the missing CSS styles issue, in my browser.
This is the file that holds the missing CSS styles shown in your earlier screenshot:
{yourwebsite.com}/wp-content/uploads/astra-addon/astra-addon-dynamic-css-post-116.css
You'll see the page as in your screenshot if for some reason this file is not loaded correctly. And since this file is not coming from Toolset plugins, if this continues, it would be best to report it to the Astra theme's support.