[Plugin: wp Time Machine] Install dir problems

I’m having some problems with this (wonderful) plugin. In the file wpTimeMachine_options.php, located at wp-content, i setup the wp_install_dir to some value ( “my-dir” ) and, after one successful cron run, the variable resets its value to null:
define( ‘wp_install_dir’, ” );

And the cronjob gives me this error:
<b>Warning</b>: require(/****/wp-config.php) [function.require]: failed to open stream: No such file or directory in <b>/******/wp-content/plugins/wp-time-machine/includes/wpTimeMachineCron.php</b> on line <b>10</b>

<b>Fatal error</b>: require() [function.require]: Failed opening required ‘/********/wp-config.php’ (include_path=’.:/usr/lib/php:/usr/local/lib/php’) in <b>/*******/wp-content/plugins/wp-time-machine/includes/wpTimeMachineCron.php</b> on line <b>10</b>

?What process changes this variable for me? ?Where it’s defined, outside this file (if it is)?

https://www.ads-software.com/extend/plugins/wp-time-machine/