I have an FTP website running on IIS 8.5 on Windows Server 2012. I can connect just fine if I disable SSL. However, if I enable SSL, the server keeps sending weird replies. Here's the log from FileZilla:
Status: Connecting to XXXXXXX:21... Status: Connection established, waiting for welcome message... Response: 220 Microsoft FTP Service Command: AUTH TLS Response: 502 AUTH command ok. Expecting TLS Negotiation. Command: AUTH SSL Error: Could not connect to server
Note how it says "502 AUTH command ok". I'm no expert on FTP, but from what I've seen on the internet, the status code should be 234, not 502. IIRC 502 actually means "command not implemented", which is not what the rest of the comment says. What's going on here?
I got a bit creative with this problem. First, I tried connecting to my own server on my own laptop. This has worked for over a year now, so no wonder it worked. Then, I moved my laptop to my workplace and connected it to the network there. I tried connecting to my own server again, I got the same error as above. I disconnected my laptop from the company's network and immediately tried again, this time via my phone's LTE connection. Guess what? Connection successful. It has to be something in the company's network that's preventing the connection... A firewall? A misconfigured proxy? Is this even possible? Any ideas on how to find the culprit?