I Just had the same problem and fixed it.
Yesterday I made a fresh install on rasbian image – usual install of opensprinkler, all updates, all worked fine.
Today I was disabling IPv6 on startup command.txt and sysctl. after reboot opensprinkler was giving me that error message.
Now i did enable ipv6 again and now it is loading again.
Funny thing I didn’t recon it is running only on ipv6 localhost. I am not sure but i belive i saw the process yesterday on ipv4:8080 too. Some commented there may be an issue on netstat. Personally I think there is a glitch when IPv6 is not available.
netstat -l -n -p Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 497/sshd tcp 0 0 0.0.0.0:443 0.0.0.0:* LISTEN 607/nginx: master p tcp 0 0 0.0.0.0:5900 0.0.0.0:* LISTEN 528/vncserver-x11-c tcp6 0 0 :::22 :::* LISTEN 497/sshd tcp6 0 0 :::5900 :::* LISTEN 528/vncserver-x11-c tcp6 0 0 :::8080 :::* LISTEN 1350/OpenSprinkler
PS: Running nginx as a simple reverse proxy to get SSL.
So just check if ipv6 is enabled.
(I used the instructinos on https://www.raspberrypi.org/forums/viewtopic.php?t=138899 to disable and reenable it).