Verifying game files only applies to games installed in the Steam directory. Your savegame is located in %appdata%7daystodieSaves. Find your save, open the folder’s properties, and see if you can restore previous versions.
You can try renaming the.bak file for that server to the save file’s name and deleting the main save file, but based on what I’ve seen on this site, the official forums, and other sites, that error usually means that the server’s save is toast and corrupted beyond use.
YouTube video which may help you – https://youtu.be/wkteeH5gR6s