Does anybody know what application puts this icon in my top bar?

It is put there from your System Preferences > Network panel, and a (Virtual Private Network Internet Protocol Security) VPN IPSec configuration panel that offers a checkbox to show that icon in the menu bar. It is there to provide a secure connection to a pre-configured VPN server. In your case, there is an incomplete setup of the VPN in your Network Preferences, and it might be as simple as the wrong server address, your account name, or even the password.

I have the same VPN icon on my menu bar to use the built-in macOS Cisco VPN security to connect through my local university via an alumni account. When configured properly, you would see the following where my Network Preference VPN config is named VPN (IPSec):

In the macOS User Guide, see Change options for L2TP over IPSec VPN connections on Mac.

You will need access to a compatible VPN server that provides you with the server address, account, and ability to set your password. In my case, the university provided me as an alumnus, a downloadable file that when I double-clicked it, proceeded to create the Cisco IPSec network device, and populate it with all of the relevant data to make a university VPN connection.

A VPN solution may be free (as in my case) or paid depending on the provider of the VPN service. I trust the local University IT staff.

Sorry to talk about an another oldest post closed by Apple without answer, ChangeAgent, have you find something about "[IGFB][LOG  ][DISPLAY   ] [Short_Pluse] FB2 port = 3" I have the same issue on Big Sur and worse, ssh and ard servers crashed after a while without use of the MacMini, I used too for a blind server. I think about an issue between Big Sur and Brew but not sure. Thx :)