php5-fpm segfault error 6 in libpcre.so.3.13.1

[Lucio Crusca <lucio@sulweb.org>]

Hi all,

I've a problem with a stretch/amd64 web server. Since two hours ago, I 
get the following in dmesg and syslog:

Jun  9 11:21:01 weber kernel: [  331.716831] php5-fpm[1245]: segfault at 
7ffcb4febf70 ip 00007f2e5e8d629a sp 00007ffcb4febf60 error 6 in 
libpcre.so.3.13.3[7f2e5e8c2000+72000]

which in turn results in a http 503 status code. However that happens 
only for one of the two VirtualHosts that make use of php5-fpm.

I've already tried lowering  pcre values in /etc/php5/fpm/php.ini:

pcre.backtrack_limit=100
pcre.recursion_limit=1000

and restarting php5-fpm (and the whole server for that matter), but 
nothing changed. The server has 8GB of RAM memory and

# ulimit -s
8192

What could I do to find the cause?