do_ypcall: clnt_call: RPC: Unable to receive; errno = Connection refused

I had been running a time-consuming background program via a bash script on a linux server. In the same bash script, I set the notification by "set -o notify" so I could know when the background job was done.

Probably yesterday night the notification popped out to say the background job was done, and after that there were following error messages, which I am not sure if occured immediately after the background job finished or a while later, since I was not at my terminal at that time:

do_ypcall: clnt_call: RPC: Unable to receive; errno = Connection refused

do_ypcall: clnt_call: RPC: Unable to receive; errno = Connection refused

do_ypcall: clnt_call: RPC: Unable to receive; errno = Connection refused

I also redirected the stdout output of my background job to a log file which seems to say the program was not finished as expected but terminated midway.

Could you explain the meaning of the error message? If possible,what kinds of problem could I possibly met? Could it be that the administrator placed some restriction on the resources that I can use on that server?

Thanks and regards!

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UPDATE:

the same hard drive is mounted via NFS over several servers including the one mentioned above. I just found a similar but different error occur on the other server:

do_ypcall: clnt_call: RPC: Timed out

This error and the previous error seem not affect the programs that are running. BTW my program has IO operations.