they are just text files.. yeah if its for http auth it needs the correct path to the htpasswd file

(and obviously the htpasswd files must be readable by the apache process)

played with it manually years ago .. guessing its still much the same

I think from memory apache comes with a tool to create them

(and there were also scripts about that could also write/manage them)

guessing in this case nagios writes them?

guessing the problem might be just permissions or an incorrect or a missing path in .htaccess or the conf files.