I'm experiencing this error as well, but I know it's not user credentials. I can successfully connect via FreeFTP and FileZilla.
However, I ran into some problems with both FTP programs, then eventually solved them. However, Flare still won't connect and gives me this error.
Details: The Server is an Azure-based server/folder. I didn't set it up so I don't more than that.
[*]FreeFTP: Would successfully connect and I could see folders and make folders. But I couldn't write any files. I forget what I changed but I eventually figured it out.
[*]FileZilla: Would timeout during connection -- it would log in successfully, but timed out when trying to read the directory. I checked settings and eventually zeroed in on its default setting (Explicit FTP). I set it to Plain FTP for encryption and the problem was solved.
Unfortunately, although I got both FTP clients working fine, Flare still craps out with the same error (550). I've tried re-entering credentials and so forth, but nothing works. I'm guessing there are some settings or something "under the hood" in Flare that are causing a problem?
Anyway, if anyone has any information on this, I'd appreciate it.
I can live with using an external FTP program, but it would be nice if I didn't have to.