Proxy_fcgi:error (70008)Partial results are valid but processing is incomplete. AH01075

I have a server running with:

• Ubuntu 16.04
• Apache 2.4.18
• WORKER-MPM
• PHP 7.0.8-0ubuntu0.16.04.3
• PHP-FPM
• OPcache 7.0.8-0ubuntu0.16.04.3

On the browser there is an ajax script that each 5 sec sends a query to a php file to update a timestamp on the DB, this script works well on other servers, but here with not so many users it log the following error:

[Mon Dec 05 09:11:39.575035 2016] [proxy_fcgi:error] [pid 7831:tid 140159538292480] (70008)Partial results are valid but processing is incomplete: [client 172.30.197.200:64422] AH01075: Error dispatching request to : (reading input brigade), referer: http://10.200….file.php

I have no idea what it is and how to fix it. I have searched the entire web and I didn't find much, any hint would be appreciated.

Edit 1:

I switch the error mode to debug and the full log for the error is this:

[Wed Dec 07 08:55:13.465599 2016] [authz_core:debug] [pid 5461:tid 139687427467008] mod_authz_core.c(809): [client 172.31.42.163:54432] AH01626: authorization result of Require all granted: granted, referer: http://10.200.200.214/sala.php?sala=Unica

[Wed Dec 07 08:55:13.465613 2016] [authz_core:debug] [pid 5461:tid 139687427467008] mod_authz_core.c(809): [client 172.31.42.163:54432] AH01626: authorization result of <RequireAny>: granted, referer: http://10.200.200.214/sala.php?sala=Unica

[Wed Dec 07 08:55:13.465634 2016] [proxy:debug] [pid 5461:tid 139687427467008] mod_proxy.c(1160): [client 172.31.42.163:54432] AH01143: Running scheme unix handler (attempt 0), referer: http://10.200.200.214/sala.php?sala=Unica

[Wed Dec 07 08:55:13.465640 2016] [proxy_fcgi:debug] [pid 5461:tid 139687427467008] mod_proxy_fcgi.c(879): [client 172.31.42.163:54432] AH01076: url: fcgi://localhost/var/www/html/sala.server.php proxyname: (null) proxyport: 0, referer: http://10.200.200.214/sala.php?sala=Unica

[Wed Dec 07 08:55:13.465652 2016] [proxy_fcgi:debug] [pid 5461:tid 139687427467008] mod_proxy_fcgi.c(886): [client 172.31.42.163:54432] AH01078: serving URL fcgi://localhost/var/www/html/sala.server.php, referer: http://10.200.200.214/sala.php?sala=Unica

[Wed Dec 07 08:55:13.465658 2016] [proxy:debug] [pid 5461:tid 139687427467008] proxy_util.c(2160): AH00942: FCGI: has acquired connection for (*)

[Wed Dec 07 08:55:13.465663 2016] [proxy:debug] [pid 5461:tid 139687427467008] proxy_util.c(2213): [client 172.31.42.163:54432] AH00944: connecting fcgi://localhost/var/www/html/sala.server.php to localhost:8000, referer: http://10.200.200.214/sala.php?sala=Unica

[Wed Dec 07 08:55:13.465668 2016] [proxy:debug] [pid 5461:tid 139687427467008] proxy_util.c(2250): [client 172.31.42.163:54432] AH02545: fcgi: has determined UDS as /run/php/php7.0-fpm.sock, referer: http://10.200.200.214/sala.php?sala=Unica

[Wed Dec 07 08:55:13.465735 2016] [proxy:debug] [pid 5461:tid 139687427467008] proxy_util.c(2422): [client 172.31.42.163:54432] AH00947: connected /var/www/html/sala.server.php to
httpd-UDS:0, referer: http://10.200.200.214/sala.php?sala=Unica

[Wed Dec 07 08:55:13.465771 2016] [proxy:debug] [pid 5461:tid 139687427467008] proxy_util.c(2701): AH02823: FCGI: connection established with Unix domain socket /run/php/php7.0-fpm.sock (*)

[Wed Dec 07 08:55:13.480503 2016] [proxy_fcgi:error] [pid 5461:tid 139687427467008] (70008)Partial results are valid but processing is incomplete: [client 172.31.42.163:54432] AH01075: Error dispatching request to : (reading input brigade), referer: http://10.200.200.214/sala.php?sala=Unica

[Wed Dec 07 08:55:13.480533 2016] [proxy:debug] [pid 5461:tid 139687427467008] proxy_util.c(2175): AH00943: FCGI: has released connection for (*)