For some reason, whenever I try booting up the new version, a popup appears saying "Zandronum.exe not found". I've tried deleting and redownloading it multiple times by now, and it seemed to work fine the first day I used it, but it hasn't been working ever since this morning.

idk try the archive version

This probably means that your antivirus is quarantining the Zandronum file, which is a pretty typical issue. You need to set a whitelist to allow it. Using the archive isn't likely to fix it.