As part of a hydraulic lifter replacement, I had to unscrew the tensioner a bit, naturally. It leaked some oil that I have paid no attention to (idiot).
Long story short, the engine doesn't start now and the chain does a weird creak when pulled (I put the car in gear and pulled it)
https://www.youtube.com/watch?v=0bCx8nf4pHw
I will not have the chance to see the car again until a day later, with some friends who are real mechanics unlike me, but I like to be useful and investigate. I think the tensioner is broken, since none of the multiple guides and step-by-step videos I followed showed or mentioned any leak. I feel like I should outright buy a new one from INA, fill it up and install, instead of trying to fix this one. The engine has 600,000km and 35 years, works perfectly, no issues what so ever. Don't want to risk it all on a shut tensioner that may have only worked because it was never disturbed.