Error code 500 when serving from cache

[guy1976]

Hi 

I'm getting random error code 500 when serving from cache, here is an
example: 

1554735637.102 10.210.44.205 57635 "GET
kCache/.../seg-101341803-s32-v1-a1.ts HTTP/1.1" 830 "" "Mozilla/5.0 (Windows
NT 10.0; WOW64; Trident/7.0; rv:11.0) like Gecko" 200 897748 0.052 HIT -
https 
1554735766.426 10.210.184.156 57413 "GET
kCache/.../seg-101341803-s32-v1-a1.ts HTTP/1.1" 602 "" "Mozilla/5.0 (Windows
NT 10.0; WOW64; Trident/7.0; rv:11.0) like Gecko" 500 0 0.000 HIT - https 

Unfortunately nginx error log doesn't show anything, any idea why? 

Thanks 
Guy. 

I'm running nginx/1.8 
here is the config: 

proxy_cache_path /cache/live levels=1:2 keys_zone=one:20m inactive=1d
max_size=5512M ; 


location ~ ^/kCache/(?<url>.*(?<cache_key>(?<=/kCache/).*).*)$ { 

proxy_cache_key $cache_key; 
proxy_pass http://proxy_backend/$scheme://$url$is_args$args; 
}

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,283716,283716#msg-283716