Checked ulimit on the server.
It is the same as on other servers running the same application, without the problem.
ulimit -a
time(seconds) unlimited
file(blocks) unlimited
data(kbytes) unlimited
stack(kbytes) 8192
coredump(blocks) unlimited
nofiles(descriptors) 256
vmemory(kbytes) unlimited
The application provided by a third party vendor runs 7x24.
The FTP error occurrs randomly, the application not cycled or restarted after the error. It will process several requests and have an error again, n files later.