Couldn't download the specific K2 Five (5.3) Fix Packs from the download list!

Hi,
why do you want to download such old fix packs? You should install the latest cumulative update (November 2020) and then the fix pack for it.
And due to possible problems, see article, the first thing to do is to install the April 2020 cumulative update.

https://community.nintex.com/t5/Technical-Issues/K2-Five-5-3-Nov-2020-Cumulative-Update-does-not-support-updating/ta-p/210262

https://community.nintex.com/t5/K2-Five-Cumulative-Updates-and/K2-Five-5-3-November-2020-Cumulative-Update/ta-p/126147

https://community.nintex.com/t5/K2-Five-Cumulative-Updates-and/K2-Five-5-3-November-2020-Cumulative-Update-Fix-Packs/ta-p/126397

https://community.nintex.com/t5/How-To/How-to-determine-the-installed-K2-software-version-updates-and/ta-p/123316