Whenever I try to open a torrent from nyaa.se dobule clicking it from the Search page, or just when I click 'download' it gives me an input/output error saying 'The torrent file doesn't exist', even though I can download it normally by opening the webpage clicking the button 'Go to description page'.

Probably it was a plugin issue, right? So I tried to update the plugin, but nothing changed, actually now after the update qbittorrent itself became laggy as fuck, it uses like 60% of the cpu for no reason and it became totally unusable.

Win7(x64), qBittorrent 3.3.3

That plugin isn't official. Try latest version from here: https://github.com/qbittorrent/qBittorrent/wiki/Unofficial-search-plugins
If it still doesn't work email the author, he has his email in that page.

@sledgehammer999 I downloaded from that link even before.

After a reboot of the system the lag issue seems to be solved, but I still have the same error as before when I try to download a torrent from the search list.

It's strange the version of the plugin isn't displayed correctly:

It's strange the version of the plugin isn't displayed correctly:

That's because the author has a space after the # characters in the .py file. Open it in a text editor and you'll what I am talking about.
Also at the top of the file you'll find the email of the author. Email him about the problem. He should be able to fix that problem.

If I am not mistaken the author is @Yukariin