the database file is just a json file .. you can try to fix it but it most probably is corrupted somehow that you will loose your data... I think the logfile tells you where the problem in the json file is. I know it won't help you with the old file but you should do backups of the file or use a mod like screepsmod-mongo that uses a real database. the included "database" always dumps the whole data when saving it .. so it is not very performant or safe for much data.