FTP Write Error (Append/Restart not permitted)

Hi Write/append causes a failure when I attempt to append to a remote file. Full reads and writes are performed successfully: i.e. : ;where blk is a block of test: write/lines full-path blk ;executes successfully read/lines full-path ; works too! write/append full-path "this is a test" ; generates the error message below: ** User Error: Server error: tcp 451 test.txt: Append/Restart not permitted, try again.. ** Where: write/append full-path "this is a test" any ideas, also would be helpful to know if code 451 is a standard tcp error or if it is rebol-originated. Thanks! Tim