I work for a web hosting company and we have a client using Contribute and upgrade to High Sierra killed her FTP connectivity.  I submitted this on the Adobe Forums but wanted to post the information here as a warning and hopefully as some use to some others troubleshooting this issue.

I tried so much via a chat session and in the end remote connected only to discover nothing I did would work.  At one point I was able to get a server error and it showed the server name with [X][X][X][X][X][X][X][X][X][X][X][X] after it which I could only surmise as null characters.  I can only assume that they were there all the time but High Sierra made them an issue whereas before they were ignored and not sent.

I cleared that out and we could then see a connection but always complained about username and password is invalid and it was not.  I was swearing (client was in chat so they could not hear me at this point and I rarely get this rattled) I then tried to start from scratch,  removed the connection and added a new one with a new username and password,  username format was not something@domain.com, however, the logs on the server side were something[X]@domain.com so another character in there,  now in usernames the @ is sometimes escaped with a / from what I know of UNIX servers and so I assume that this was being misunderstood by the server.

I then switched back to the account username that had no @domain and we them sat a valid connection and valid username.  Password, however, would never work.

We had a ? in the password so I switched to an alphanumeric password and still no go.  Went down to plain FTP so as to not complicate matters and still no luck.

I would be very careful if you consider upgrading,  have a full backup and a means to roll back your MacOS.