apr_socket_connect(): Operation not permitted (1)?

Question

Apache HTTP Server

I'm doing a benchmark of a server distributing statics (works on lighttpd/1.4.30). Benchmarking from a neighboring server using ab (Apache HTTP server benchmarking tool). It looks something like this:

# ab -n10000 -c50 -t60 -r <a href="http://domain.tld/image.gif">domain.tld/image.gif</a><br/>
This is ApacheBench, Version 2.3 &lt;$Revision: 655654 $&gt;<br/>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, <a href="http://www.zeustech.net/">www.zeustech.net/</a><br/>
Licensed to The Apache Software Foundation, <a href="http://www.apache.org/">www.apache.org/</a><br/>
<br/>
Benchmarking domain.tld (be patient)<br/>
Completed 5000 requests<br/>
<br/>
Test aborted after 10 failures<br/>
<br/>
apr_socket_connect(): Operation not permitted (1)<br/>
Total of 7943 requests completed

The situation is stable, the number of Total of N requests completed floats around the number 7500.
What is apr_socket_connect(): Operation not permitted (1)? At least on whose side is the error - not a checked host, or on the checking one? Google something does not please me with a large crowd of brothers in misfortune.