SUMMARY
The ownCloud client icon is missing (invisible?) from the systemtray. It only shows an empty spot at the place where the icon should be. The empty spot is clickable, and the click opens the ownCloud window as it should.

STEPS TO REPRODUCE
1. Log in to plasma session with the ownCloud client previously enabled to launch at startup.

OBSERVED RESULT
- icon is missing

EXPECTED RESULT
- icon is visible.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Fedora 34, oxygen-icon-theme 5.85.0
(available in About System)
KDE Plasma Version: 5.22.4
KDE Frameworks Version: 5.85.0
Qt Version: 5.15.2
owncloud-client: 2.7.4-2

ADDITIONAL INFORMATION
I recently updated from Fedora 33 to 34 and icon disappeared (using the oxygen icon theme). I can see an owncloud icon if I switch to one of the Breeze icon themes, and restart the owncloud client.

I am actually not sure if this is an oxygen bug. Possibly it is a bug in systemtray or somewhere else, but I couldn't find the proper KDE product for the sytemtray to report the bug there.