It is a Git error. Your git repo for backup might be corrupted. You may try:
- Back up all your project in the backup folder (
C:\Users\<user>\Documents\PHPMaker\Backup) first. - Delete the git folder for backup (
C:\Users\<user>\Documents\PHPMaker\Backup\.git). Note that this will delete all the backup history, so make sure you have done step 1. - Re-open PHPMaker and try again.