I had a perfectly working installation of KDE Connect on Linux Mint Cinnamon 20.3, until one day I decided to try the KDE Plasma environment on the same LInux mint installation.

After installed Plasma Desktop environment, Connect continues to work well on Plasma, but refuses to show me a tray icon in Cinnamon.

I have no idea where to find error logs, if any. Please advice.

If further information is required to debug this, please specify how to find it.

I don't use Cinnamon, maybe ask in a Cinnamon forum? I doubt the error is caused by Plasma.