I figured out the problem after hours of research and it’s just embarrassing…
I checked the invoice from my jack adapters to see if they had any bad reviews or anything else online. Instead of finding those, I found out that Thomann never sent me 3.5mm - 6.3mm jack adapters, but an RCA to 6.3mm jack. I used my RCA cables to try it and its working just fine, so the hardware is good! Got another REAL adapter today and some 6.3mm cable are on their way!
Thanks again everyone for helping me figuring that out 
I took a few days off to discover now everything this wonderful piece of technology has to offer!