You can experience this problem on Radar/Yantar (common) and on other locations (very rare). Cause of the problem - civil zombies, which in fact are monsters and monolith/zombied stalkers shared the same "species", that led game engine to transfer hit memory signals from monolith/zombied stalkers to the civil zombies. Civil zombies are monsters, and they just haven't engine support for hit memory, so then player saved game until any hit memory signal received by zombies are expires - this savegame is became unusable. This problem can be avoided by installing this small fix. Just download archive and extract it's contents inside main game folder, there the mod is installed, overwriting current files. Start of new game is required for this fix to take effect.