" I am feeding the LNB with 13V, supposed to be vertical polarisation…
Anyway, I'll keep searching how I can improve this…"
Feeding the LNB with +13V does not ensure vertical polarisation alone.
If you feed +13V but mechanically rotate 90 Degrees you still get horizontal polarization !!
Personally i first stumbeled about DC-feed problems cause my homebrew bias-T
had not enought bandwith and was blocking everything above 500MHz - check your IF path aswell