Getting error AH00646 (Error wrting to logger) in WAF log

Greetings, 

I regularly get error entries in our WAF log looking like this:

2017:07:06-09:18:51 bchzhutm01 httpd[32187]: [log_config:warn] [pid 32187:tid 3752549232] (32)Broken pipe: [client 2.166.63.131:61966] AH00646: Error writing to |/bin/logger -p local1.info -t httpd

Any idea what this means or how to fix it? It's logged for various client IP addresses.

Thanks,

Bernhard