Try narrowing the problem down. Go to the website via computer, throw a flash drive in that's formatted W/ Fat32, (if your computer won't format fat32, the fire tv will, do it there) and go to the website and save the repo or stalker plus to the drive. Eject it. Pop it in the fire. Open Kodi, go to install from zip, look for the name of your drive and try installing that way. If it won't, you know it's probably not on their end, more likely yours. Since you say everything else downloads fine...... This should work.
Edit: missed the bit about trying on a PC . Sorry.