Sorry, I also noticed lots of this lines in virtual host error log:
[Sat Feb 13 12:34:11 2016] [info] [client xx.xx.xx.xx] FastCGI: client stopped connection before send body completed[Sat Feb 13 12:34:11 2016] [debug] mod_headers.c(743): headers: ap_headers_output_filter()[Sat Feb 13 12:34:11 2016] [debug] mod_headers.c(743): headers: ap_headers_output_filter()[Sat Feb 13 12:34:11 2016] [debug] mod_headers.c(743): headers: ap_headers_output_filter()[Sat Feb 13 12:34:11 2016] [debug] mod_headers.c(743): headers: ap_headers_output_filter()[Sat Feb 13 12:34:11 2016] [debug] mod_headers.c(743): headers: ap_headers_output_filter()[Sat Feb 13 12:34:11 2016] [debug] mod_headers.c(743): headers: ap_headers_output_filter()[Sat Feb 13 12:34:11 2016] [debug] mod_headers.c(743): headers: ap_headers_output_filter()[Sat Feb 13 12:34:12 2016] [debug] mod_headers.c(743): headers: ap_headers_output_filter()[Sat Feb 13 12:34:12 2016] [debug] mod_headers.c(743): headers: ap_headers_output_filter()....[Sat Feb 13 12:37:08 2016] [info] [client yy.yy.yy.yy] (104)Connection reset by peer: core_output_filter: writing data to the network
