Yes, I meant updating to a newer version. Even if you make a backup of config_defaults_inc.php before updating MantisBT you can get problems. If a newer version of MantisBT introduces new configuration options in config_defaults_inc.php and you copy your backup over it, the new options are not set to an initial value. This might lead to various issues.
Please use Search before posting and read the Manual Use Mantis2Go to try MantisBT on Windows or to reproduce issues