Also if you don't have other PC or source with 5.1 output (like DVD/Bluray player) to use to test speakers, you could change black cable to green jack in PC to test if that gives sound from rear left/right speakers.
Alright this is a bit embarrassing.I spent another few hours trying to do the things that you suggested and looking for other solutions myself when I decided to have another look in the manual. The speakers have a few different modes: 2.1 stereo, 4.1 stereo and 3D surround. I assumed that 3D surround was just a marketing name Logitech gave to 5.1 surround, but apparently it's a mode that simulates surround in stereo. so in order to use true 5.1 surround I had to disable all modes. now everything works absolutely fine.
Thank you so much for your help and for your time, because otherwise I'd probably still be messing around with with the pink and orange connectors.
So to anyone who might have the same problem: Keep cycling through effect either on your remote or on the the control hub itself until there's no orange light anymore next to 2.1, 4.1 and 3D