It seems to be a server error, from a server called vsftpd.
It appears to mean that the server has run out of sockets. This could happen if a lot of clients are downloading or uploading files during a short period. It takes a few minutes for used sockets to be freed up. Apparently earlier versions of vsftpd were more prone to this error.
See the
the vsftpd FAQ.
"Q) Help! I'm getting messages along the lines of 500 OOPS: vsf_sysutil_bind
when trying to do downloads (particularly lots of small files).
A) vsftpd-1.2.1 should sort this out."