Even though both servers are configure the same I’m getting the error message
Server crawl-01 gives error “NRPE: Unable to read output” while server control-01 has no problem returning output
What exactly does this mean?
Sounds like NRPE: Unable to read output means that whatever was returned, is not able to be read.
Try the command by hand and see what you get for output. Then simply change the hostname to a different hostname, and see if that is any different. If so, then the problem is on the remote host setup for nrpe.