This is probably due to file permissions issues.
1) delete the webassist/themes/temp_editing folder
2) use an ftp program like filezilla to update the permissions of the webassist folder and all of its subfolders and files to: 755
3) replace the admin/common/properties.php page with the one attached
4) replace the webassist/plugins/styler/styler.php page with the one attached
Then try it again and see if it works.