Problems after upgrade to 2.0.4.21 appears to be a major issue for “FTP (Alternative)” users on Linux or similar UNIX-like FTP servers (maybe including those embedded in a device such as a NAS or a router). Fortunately the alternative to “FTP (Alternative)” is plain “FTP” 
but possibly some can’t switch to that. Usage statistics shows about as much “FTP (Alternative)” as “Amazon Cloud Drive”. “FTP” is far higher.