Indicator icons do not appear after upgrade to Ubuntu 17.10

I've encountered the same problem. I tried to remove the indicator-application-service completely, and see, the problem went away! I guess coming from 16.04 leaves some Unity-related stuff on my machine that is not only obsolete but prevents the AppIndicator extension from properly working.

What to do:

sudo apt remove indicator-application

I have the same problem with 17.10. At least the following command helps here to bring the icons back until the next reboot:

killall indicator-application-service

Edit: This workaround is running fine here without using any gnome-extension like "Topicon-plus" or "Kstatusnotifieritem/appindicator support".

As suggested here, try removing all obsolete indicators if any still present

indicator-application
indicator-appmenu
indicator-bluetooth
indicator-common
indicator-datetime
indicator-keyboard
indicator-messages
indicator-network
indicator-power
indicator-printers
indicator-session
indicator-sound
indicator-transfer
indicator-transfer-download-manager

Just execute this command in a terminal:

sudo apt purge indicator-application indicator-appmenu indicator-bluetooth indicator-common indicator-datetime indicator-keyboard indicator-messages indicator-network indicator-power indicator-printers indicator-session indicator-sound indicator-transfer indicator-transfer-download-manager