Open your project; in the menu go to Playback > Delete Render Cache > All. Done.
If you have multiple projects and you don’t know which one (or more) of them is eating up your disk space – open them one by one and perform the abovementioned operation for each of them.
It’s Davinci Resove v16 in my case, but the issue existed before this version as well.
I’ve just freed up 35GB of space that will let me wait a few days before I have to buy a new hard drive anyway 🙂