Hey there 👋!
I encountered the same error as you and for me were 2 ways to see from what was it.
1. If you are using cloudflare, make sure is not proxied. ( For me this was a problem, not showing the css at all, I use custom Nginx protection and it was fighting with Cloudflare, by disabling proxied, worked again ).
2. Data-Store path is incorrect.
- What I did? Logged in AdminCP, and tried to find "clear caches" it was hard but I did it, after that some of the CSS came back so to say and I updated the data-store path and everything worked again.
Follow these links to update data-store path:
1. Create New (https://yourdomain.com/admin/?app=core&module=overview&controller=files&do=configurationForm
2. Edit Current One: (https://yourdomain.com/admin/?app=core&module=overview&controller=files&do=configurationForm&id=1)