It turns out that Polish characters, which I used, are not recognized (Latin symbols only). I filled out the form again (without Polish symbols) and this time everything was fine. Credits to Oleg Onischuk
In the very beginning, I did the way you wrote but after I got the error message I decided to look into the file. I was surprised by its size - 1 byte. That would be kind of strange to name something like that a license file/key.
I downloaded StarWind Virtual SAN Free along with the license key. When I'm opening the key via whichever text editor I get the following: j (a single letter). Then, when I'm trying to use the key I get the following error: Failed to check license signature. I've tried downloading the key three time...