It says the Hangfire server that was handling the job is no longer active, so once another Hangfire server is started to take its place it will retry it after a timeout. Pretty self explanatory.
Ensure your Hangfire server is till running, and is kept running correctly.
I personally would advise not running the Hangfire server in an IIS, since it seems difficult to configure an IIS to handle running a PROGRAM without stopping it every few minutes because of site inactivity.
It is much simpler to run the Hangfire server as a Windows Service using Topshelf or something similar. Even running it as a console app seem more reliable than an IIS.