c# - Visual Studio loses ability to attach to Unity, why?

Question

I'm using Visual Studio with Unity. In general in VS I can simply click "Attach to Unity" and it will build the solution and indeed attach to Unity. Sometimes, however, a project loses the "ability" to do so, what means that suddenly clicking the button will only build the solution and not attach to Unity. I really would like to know what's happening there as I'm not aware of doing anything that could result in such a behaviour.
There are some parameters that might have an effect:

• it happens only for some Unity projects: a project is either "infected" or it happens never (on my watch)
• if it starts happening it will always happen until I reboot the PC (closing VS and Unity is not enough)

Thank you

Answer 1

For me "Attach to Unity" disappeared from the toolbar.

But I still have a button in Debug -> Attach Unity Debugger

Hope it will help someone.

Visual Studio Community 2019 16.7

Unity 2020.1.2f1