I’m probably already out of my depth here, but taking cues from this fairly old thread:
there might be an issue with the nginx component of this.
I assume, you’ve looked at the general log after boot up:
/opt/rockstor/var/log/rockstor.log
next you could also execute the command:
systemctl status -l rockstor-pre rockstor rockstor-bootstrap
if we’re lucky it will point us in the right direction. If it is indeed the nginx, you can check the logs as pointed out in the thread from above:
/var/log/nginx/error.log