Alright, so after full 2 days of screwing with this bug, trying every possible combination of options, rebooting my rpi 100+ times, restarting configs, etc etc… I can proudly say I finally made some progress, and figured out the cause of this problem.
So, I noticed that if I
rm -r .kodi
and restart rpi, then go to settings and check “Allow remote control via UPnP” (because its disabled by default), everything mysteriously works correctly.
So, then I copied back my old .kodi folder back and started deleting files one-by-one, to see when it will stop working again. And sure enough, it was the last thing I could’ve think of. It was the .kodi/userdata/Database folder.
TLDR: If I: rm -r .kodi/userdata/Database/, dlna starts working again as it should.
But, it seems to be only temporarily - after a while (a few hours), it stops working again, and you have to delete Database folder again.
I will try and delete file inside Database folder one by one, to see which file exactly is the problem, and will update you when I have time again to screw with this.