If you’re on Windows, and you use Filezilla to connect to an IIS server using FTPs to a self-signed certificate, the latest versions (3.24+) will throw the following error:
GnuTLS error -48: Key usage violation in certificate has been detected.
Could not connect to server.
Check this thread on Filezilla’s forum for more details. Basically, either turn off those self-signed certificates and use plain FTP, or, revert back to an earlier version of Filezilla. (Or, wait until Microsoft issues an update… [tumbleweed])
(Older versions of Filezilla here.)