Thanks for your reply.
I was about to write that I have read somewhere online that since some firmware update pairing with Windows is not possible anymore. Also not by that web-bluetooth script found on Github.
But then I tested again on my laptop which has newer, built-in bluetooth and it just paired instantly! So I forgot about that old bluetooth dongles will not work. I´m gonna replace my old MSI dongle which has version 2 or so. ^^
That´s why I favour communicating with people instead of just using a search machine!