Another option for the steam thing is adding the following to the additional launch settings:

gamemoderun %command%

I’m pretty sure Mint comes with it already set up, and it’s made the difference between “working” and “not working” for me before.

Wow, not even linking the patched version

I’m not exactly sure what might be causing your problem, but I’m assuming you’ve tried purging & redownloading KDE Connect?

Another one would be trying to kill whatever process it thinks it running KDE Connect. You could either find it the normal way, or this link might help if you can’t easily find the process you need to kill.