I've moved the modem outside the cabinet and used a short cable (that cable on eBay for those with problems or w/e, with that ferrite bead integrated), with a micro-filter. Not sure if it's the cable, the micro-filter or merely just the way the line has now become but the Hlog graph looks different.
https://i.imgur.com/cpHhwvR.pngA nice wave like line on the first downstream band.
I'll try switching to a simple flat RJ11 cable to see if it disappears, then if not I'll try switching back to the MK3 SSFP. Interesting though.
EDIT: The mentioned cable from eBay caused that wave like line it seems, now it's smooth with a flat RJ11 cable. FEC errors are still there however, in a large amount. I hope the engineer decides to try a 'circuit recalc' and it should hopefully reveal the stupidly high amount of error seconds I had (especially with the amount of FEC errors I'm getting still). On the other hand, re-syncing made my line attenuation return to normal.