The server's certificate is unknown. Please carefully examine the certificate to make sure the server can be trusted.|
It is neither an error nor a problem. FileZilla just doesn't use the OS certificate store (which might become compromised), instead, it follows the TOFU model (TOFU = Trust On First Use). That means, at least upon first contact to every new server, you will get that popup, to carefully check and verify the certificate. If you trust it, click the button (+check the box for permanent trust). With permanent trust, you won't be bothered again for that exact certificate until it expires.
Just check these two checkbox.
Trust this certificate and carry on connecting? YES
Always trust certificate in the future sessions. YES