according to Oracle Support note 467720.1:
The problem is caused by the list command (ls) command executed by the FTP adapter returning an FTP error code of 550 (Requested action not taken: file unavailable). This error code will cause the adapter to be shut down, failing the FTP adapter operation.
This can be overcome by following these steps:
(1) File name patterns should be provided in the FTP adapter wizard field "Include Files with Name Pattern", for example such as can*.txt, i.e. it needs to have some characters and then * in the pattern, this solution will not work if providing only * as the file name pattern.
(2) A file with a different pattern that does not match with the pattern provided above should always be present in the input ftp directory. Such as a file called test.txt is put into the ftp directory and the BPEL/ESB process file pattern name in "Include Files with Name Pattern" is can*.txt, as in (1) above.