Lately on plasma I could not do the change of my avatr, I found the solution on:
if I can be of help I put the solution.
Until the developer’s fix it, this is reported to work:
The issue is with the service file for AccountsService:
sudo nano /usr/lib/systemd/system/accounts-daemon.service
this is set to PrivateTmp=true
setting it to: PrivateTmp=false
and reboot will solve it