Hi, everyone! :)
Since the problem occures often and I had it many times, after finding a way out of this, I thought it would be useful to share it.
If the Indicator Plugin is missing from your list, you can install it with the following:
sudo apt install xfce4-indicator-plugin
Video: Fixing the XFCE Tray icons problem
https://www.youtube.com/watch?v=HsbWaeYCHB8
Best regards! :)