After uninstalling ISPConfig3 so as to reinstall it, nginx gives the error: nginx: [emerg] unknown "no_cache" variable [FAILED] ...and refuses to start. After checking the error-log for nginx this happened exactly when ISPConfig3 was removed. My hunch is that somehow the config's for the vhost has something to do with it, but not sure why since all have been removed. Working on the problem now, we have isolated nginx to start with only nginx.conf and it starts so this eliminates anything in the core and recompiling is not necessary. Now we are working through some configurations in some of the sites that were operating "outside" of ISPConfig3. Bewildered as to why because everything was working perfectly on those sites and no recent work was being done on any of the configs for those server blocks.