Happy New Year! And a pray for the fires all over the world, especially Australia, to stop!
Finally, I did it again! To remove this error 503 you need to make a clean uninstall of Immunet first and then reinstall it!
Uninstall, clear all data. After restarting, delete folders of immunet on program files and data.
After, running regedit.exe, search for "immunet" delete these keys and subkeys... (I deleted some root keys also (where I see that it is only for immunet).
Then reinstall! I didn't try it without registry cleaning (only reinstall after uninstalling) but you can try it first (just to not mess up with registry)
Happy new 2020