502-Auth command is not supported

Hello,

I have a connection to an FTP server located on another company.
The server is running on Microsoft FTP Service.
Today I am having a problem to connect to the server and the error is the following:

502-Auth command is not supported.
Please contact your network administrator to enable FTPS service.

I contacted the administrator of the server but he says that the server is up and running and he cannot find anything wrong with it.
I enabled the debug in logging options and tried again to get some more information.
You can find attached the log file.

Please advise.

It looks like the server does not support encryption.
But as far as I know, IIS always supports encryption. So it's strange.
Are you sure you were using encryption before?
I'd fear that you might be under MITM attack. Be cautious.

Yes the connection is the same for years I didn't changed it recently.
Hmm I will investigate with the firewall guys if it is possible to identify that.

This could be anything. I don't know what Microsoft FTP does with AUTH and they could fail or have a bug or whatever.

AUTH has to successfully open a socket/port for FTPS explicit. Although, it should be simple and then a 234 should be sent to the client and then the socket and port becomes available. But, Microsoft could do various checks or things before doing that if they really wanted and just say not supported.

It could be like Martin suggests, malicious. But, stuff happens. It won't be that in every single case.

So after investigation it seems that it was a firewall issue.
Apparently there was an update on firewall that was causing this.
Unfortunately I cannot give more details about this but I guess if someone else experience this issue it would be a good idea to involve the firewall team in their troubleshooting.

Thank you guys for your replies.

@tuccero9: Thanks for sharing your findings.