500 error for historylog.sh 🔗 link

I've got xymon 4.3.28 with apache 2.4.25 and I'm seeing a 500 error for historylog.sh with the sanitized log entry in the error_log

AH02429: Response header name '3360 2017-10-13 19' contains invalid characters, aborting request, referer: 
http://my-site.site/xymon-cgi/history.sh?HISTFILE=my-site.site.procs&ENTRIES=50&IP=xx.xx.xx.xx&DISPLAYNAME=my-site.site

Thoughts?